Export vitePreprocess API

[bluwy]

Export vitePreprocess() Svelte preprocessor from @sveltejs/vite-plugin-svelte. (Previously exported from /preprocess subpath but now root)

experimental.useVitePreprocess is deprecated.

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Things I'm still thinking of, but could be a future problem:

1. Should we deprecate useVitePreprocess?
2. Following no1, there are frameworks like Astro that wants to preprocess with Vite by default. The user can get intellisense by adding vitePreprocess to svelte.config.js but that results in two same preprocessors running at the same time. Should we deduplicate them internally?
3. When running in a Vite dev server that has it's own ResolvedConfig, do we want to share it to the style preprocessor? If the user passes an InlineConfig to the style preprocessor, it makes config merging not possible. Also it feels a bit magical 🤔

EDIT: Answered:

1. yes.
2. Astro should deduplicate themselves.
3. now implemented in this PR. When using the preprocessor in Vite dev server, the preprocessor will use that config.

[dominikg]

yes to deprecate, the improvement in DX makes using the plugin option inferior. we can add docs how to migrate. it's experimental still so no semver constraints

[benmccann]

I'd be curious if there's been any rough benchmarking to see if this is faster. esbuild is obviously faster, but you'd also have to call out to another process which is slower (I think once per file), so I wonder if it's overall faster. If we can confirm it's faster, I wonder if this should be made the default in create-svelte?