[chore] provide Vite config via plugin

[benmccann]

Kit is now simply a Vite plugin in dev mode

We could move it to a new location and export it as @sveltejs/kit/vite, but I didn't want to move files around in the same PR where I'm refactoring things to make it more manageable to understand what's changing. I'll do that after this PR is merged in a follow up

[changeset-bot]

🦋 Changeset detected

Latest commit: f1e52dc

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 1 package

Name	Type
@sveltejs/kit	Patch

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[Rich-Harris]

Suggested change

	const config = {
	plugins: [...(vite_config.plugins || []), svelte(svelte_config), sveltekit(svelte_config)]
	};
	config.server = config.server || {};
	const config = {
	plugins: [...(vite_config.plugins || []), svelte(svelte_config), sveltekit(svelte_config)],
	server: {}
	};

[benmccann]

It's a more natural way to write it, but upsets TypeScript. TypeScript see the type annotation which says server can be optional and that takes precedence. If we want to do it the suggested way then I also need to add & {server: import('vite').ServerOptions} to the type, which seemed even uglier to me. I just noticed the way I have it written currently can be simplified though

[benmccann]

I'm going to merge this since it seems like we're pretty much agreed on the details at this point that way I can get started on the next PR. Feel free to leave comments still on either this PR or the next one and I can tweak any remaining items still

[seandlg]

This likely caused a weird ParseError: The keyword 'let' is reserved error for me. Removing a dangling vite.config.js made things go back to normal. See this discord discussion for more info.