fix: generate type definition with nullable types

[jasonlyu123]

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Fix regression introduced in svelte 4. sveltejs/language-tools#1767. This is because the type definition is now generated from the source, and TypeScript will ignore nullable types when strictNullCheck isn't enabled.

Since the types aren't in source control. Manually create a diff between before and after for record and review.

--- a/packages/svelte/types/index.d.ts
+++ b/packages/svelte/types/index.d.ts
@@ -110,7 +110,7 @@ declare module 'svelte' {
 		
 		$destroy(): void;
 		
-		$on<K extends Extract<keyof Events, string>>(type: K, callback: (e: Events[K]) => void): () => void;
+		$on<K extends Extract<keyof Events, string>>(type: K, callback: ((e: Events[K]) => void) | null | undefined): () => void;
 		
 		$set(props: Partial<Props>): void;
 	}
@@ -772,8 +772,8 @@ declare module 'svelte/compiler' {
 	 * https://svelte.dev/docs/svelte-compiler#svelte-preprocess
 	 * */
 	export function preprocess(source: string, preprocessor: PreprocessorGroup | PreprocessorGroup[], options?: {
-		filename?: string;
-	}): Promise<Processed>;
+		filename?: string | undefined;
+	} | undefined): Promise<Processed>;
 	/**
 	 * The current version, as set in package.json.
 	 *
@@ -1576,13 +1576,13 @@ declare module 'svelte/motion' {
 	 *
 	 * https://svelte.dev/docs/svelte-motion#spring
 	 * */
-	export function spring<T = any>(value?: T, opts?: SpringOpts): Spring<T>;
+	export function spring<T = any>(value?: T | undefined, opts?: SpringOpts | undefined): Spring<T>;
 	/**
 	 * A tweened store in Svelte is a special type of store that provides smooth transitions between state values over time.
 	 *
 	 * https://svelte.dev/docs/svelte-motion#tweened
 	 * */
-	export function tweened<T>(value?: T, defaults?: TweenedOptions<T>): Tweened<T>;
+	export function tweened<T>(value?: T | undefined, defaults?: TweenedOptions<T> | undefined): Tweened<T>;
 }
 
 declare module 'svelte/store' {
@@ -1652,16 +1652,16 @@ declare module 'svelte/store' {
 	 * https://svelte.dev/docs/svelte-store#readable
 	 * @param value initial value
 	 * */
-	export function readable<T>(value?: T, start?: StartStopNotifier<T>): Readable<T>;
+	export function readable<T>(value?: T | undefined, start?: StartStopNotifier<T> | undefined): Readable<T>;
 	/**
 	 * Create a `Writable` store that allows both updating and reading by subscription.
 	 *
 	 * https://svelte.dev/docs/svelte-store#writable
 	 * @param value initial value
 	 * */
-	export function writable<T>(value?: T, start?: StartStopNotifier<T>): Writable<T>;
-	export function derived<S extends Stores, T>(stores: S, fn: (values: StoresValues<S>, set: (value: T) => void, update: (fn: Updater<T>) => void) => Unsubscriber | void, initial_value?: T): Readable<T>;
-	export function derived<S extends Stores, T>(stores: S, fn: (values: StoresValues<S>) => T, initial_value?: T): Readable<T>;
+	export function writable<T>(value?: T | undefined, start?: StartStopNotifier<T> | undefined): Writable<T>;
+	export function derived<S extends Stores, T>(stores: S, fn: (values: StoresValues<S>, set: (value: T) => void, update: (fn: Updater<T>) => void) => Unsubscriber | void, initial_value?: T | undefined): Readable<T>;
+	export function derived<S extends Stores, T>(stores: S, fn: (values: StoresValues<S>) => T, initial_value?: T | undefined): Readable<T>;
 	/**
 	 * Takes a store and returns a new one derived from the old one that is readable.
 	 *
@@ -1743,31 +1743,31 @@ declare module 'svelte/transition' {
 	 *
 	 * https://svelte.dev/docs/svelte-transition#blur
 	 * */
-	export function blur(node: Element, { delay, duration, easing, amount, opacity }?: BlurParams): TransitionConfig;
+	export function blur(node: Element, { delay, duration, easing, amount, opacity }?: BlurParams | undefined): TransitionConfig;
 	/**
 	 * Animates the opacity of an element from 0 to the current opacity for `in` transitions and from the current opacity to 0 for `out` transitions.
 	 *
 	 * https://svelte.dev/docs/svelte-transition#fade
 	 * */
-	export function fade(node: Element, { delay, duration, easing }?: FadeParams): TransitionConfig;
+	export function fade(node: Element, { delay, duration, easing }?: FadeParams | undefined): TransitionConfig;
 	/**
 	 * Animates the x and y positions and the opacity of an element. `in` transitions animate from the provided values, passed as parameters to the element's default values. `out` transitions animate from the element's default values to the provided values.
 	 *
 	 * https://svelte.dev/docs/svelte-transition#fly
 	 * */
-	export function fly(node: Element, { delay, duration, easing, x, y, opacity }?: FlyParams): TransitionConfig;
+	export function fly(node: Element, { delay, duration, easing, x, y, opacity }?: FlyParams | undefined): TransitionConfig;
 	/**
 	 * Slides an element in and out.
 	 *
 	 * https://svelte.dev/docs/svelte-transition#slide
 	 * */
-	export function slide(node: Element, { delay, duration, easing, axis }?: SlideParams): TransitionConfig;
+	export function slide(node: Element, { delay, duration, easing, axis }?: SlideParams | undefined): TransitionConfig;
 	/**
 	 * Animates the opacity and scale of an element. `in` transitions animate from an element's current (default) values to the provided values, passed as parameters. `out` transitions animate from the provided values to an element's default values.
 	 *
 	 * https://svelte.dev/docs/svelte-transition#scale
 	 * */
-	export function scale(node: Element, { delay, duration, easing, start, opacity }?: ScaleParams): TransitionConfig;
+	export function scale(node: Element, { delay, duration, easing, start, opacity }?: ScaleParams | undefined): TransitionConfig;
 	/**
 	 * Animates the stroke of an SVG element, like a snake in a tube. `in` transitions begin with the path invisible and draw the path to the screen over time. `out` transitions start in a visible state and gradually erase the path. `draw` only works with elements that have a `getTotalLength` method, like `<path>` and `<polyline>`.
 	 *
@@ -1775,14 +1775,14 @@ declare module 'svelte/transition' {
 	 * */
 	export function draw(node: SVGElement & {
 		getTotalLength(): number;
-	}, { delay, speed, duration, easing }?: DrawParams): TransitionConfig;
+	}, { delay, speed, duration, easing }?: DrawParams | undefined): TransitionConfig;
 	/**
 	 * The `crossfade` function creates a pair of [transitions](/docs#template-syntax-element-directives-transition-fn) called `send` and `receive`. When an element is 'sent', it looks for a corresponding element being 'received', and generates a transition that transforms the element to its counterpart's position and fades it out. When an element is 'received', the reverse happens. If there is no counterpart, the `fallback` transition is used.
 	 *
 	 * https://svelte.dev/docs/svelte-transition#crossfade
 	 * */
 	export function crossfade({ fallback, ...defaults }: CrossfadeParams & {
-		fallback?: (node: Element, params: CrossfadeParams, intro: boolean) => TransitionConfig;
+		fallback?: ((node: Element, params: CrossfadeParams, intro: boolean) => TransitionConfig) | undefined;
 	}): [(node: any, params: CrossfadeParams & {
 		key: any;
 	}) => () => TransitionConfig, (node: any, params: CrossfadeParams & {