types: allow void return in LoadOutput

[ignatiusmb]

Fixes #1606, and cleans up some more types

[dummdidumm]

Thanks!

[janosh]

Shouldn't the void | in

MaybePromise<void | LoadOutput<
	InferValue<Output, 'props', Record<string, any>>,
	InferValue<Output, 'context', Record<string, any>>
>>;

be part of LoadOutput? Otherwise a Load function like

async function load(input: LoadInput): Promise<LoadOutput> {
  // ...
}

will still raise a type error when not returning.