This allows you to pass the Svelte(Kit) config into the sveltekit Vite
plugin. Example:

```js
import { sveltekit } from '@sveltejs/kit/vite';
import { defineConfig } from 'vite';
import adapter from '@sveltejs/adapter-vercel';

export default defineConfig({
	plugins: [
		sveltekit({
			compilerOptions: {
				experimental: {
					async: true
				}
			},
			adapter: adapter(),
			experimental: {
				remoteFunctions: true,
				handleRenderingErrors: true
			}
		})
	],
});
```

Note how the `kit` namespace is at the same level as the other top level
entries; this is the only difference to the current `svelte.config.js`
layout.

If you set your options via the plugin, the `svelte.config.js` contents
are ignored, so it's either-or, not a merge.

In SvelteKit 3 we will likely make it a requirement to pass the options
via the Vite plugin instead - one less config file at the cluttered top
level of your project.

Fixes #5485

Because you can tell Vite which config to use this also fixes #13748

Related: sveltejs/language-tools#3031

---------

Co-authored-by: vercel[bot] <35613825+vercel[bot]@users.noreply.github.com>
Co-authored-by: Vercel <vercel[bot]@users.noreply.github.com>
Co-authored-by: Rich Harris <rich.harris@vercel.com>

…3031)

We're thinking about getting rid of `svelte.config.js` (or rather, soft-deprecating it): `vite.config.js/ts` is now pretty much the standard for new Svelte apps and generally a better place to put your config.

This is an important stepping stone towards that future. `svelte.config.js` was originally introduced for language-tools to read config options, now we can do the same by reading the Vite config.

With this PR, `vite.config.js` is read first / preferred if both exist, but if it does not exist or no options are retrieved through it then `svelte.config.js/ts` is used.