feat: Support svelte.config.ts

[dominikg]

This PR enables loading svelte.config.ts in runtimes that support importing TypeScript files.
In deno and bun this should work ootb, in nodejs >=22.6 you can use the --experimental-strip-types flag to allow it.

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Latest commit: 790ced2

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Preview: https://svelte-dev-git-preview-kit-13935-svelte.vercel.app/

[dummdidumm]

The lint error is from eslint which likely trips up on multiple files just with different extensions, best to ignore that directory in the config.
The other error sounds like something from the runtime, but not sure. Maybe NodeJs because the CI runs with a version which doesn't support ts files yet?

[dominikg]

ugh, conditionally passing --experimental-strip-types is weird.

changed test-others to a matrix to be able to test with 22 and 18.

In a separate PR we should look into adding node24 to the matrix tests.

[benmccann]

I'm not sure it makes sense to support .mjs and .mts file extensions. SvelteKit projects can only be ESM projects in which case you'd never use those extensions

[dominikg]

When it was only one exact filename svelte.config.js it made the code simpler not checking svelte.config.mjs too.

Now that this PR adds checking multiple extensions, its very easy to allow mjs and mts and those are valid even in projects where "type":"module" is in package.json. They also enable custom setups where type is commonjs (eg sveltekit app embedded in a node app package).

[benmccann]

To me it's nothing about the simplicity of the code, but whether we want to encourage bad practices. There is no reason a SvelteKit project should ever lack "type": "module" or use .mjs/.mts that I can think of

[dominikg]

removed mjs and mts, lets see if someone asks for it. I know there's a few that use vite-plugin-svelte with svelte.config.mjs but SvelteKit can have different constraints.