feat: add support for svelte config ts/mts files

[craig-jennings]

Fixes #2834

This is a follow-up to #2804 that was closed for some reason. It'd be great to get this in because I think it's the last blocker to being able to fully adopt svelte.config.ts files in a project. The only adjustment I made was adding @jrmajor's recommendation to remove the flag for Node 22.18 because it was enabled by default on that release

[changeset-bot]

🦋 Changeset detected

Latest commit: 3b8013f

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

This PR includes changesets to release 2 packages

Name	Type
svelte-language-server	Patch
svelte-check	Patch

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[jasonlyu123]

The flag is still needed. The language server is using Node.js version bundled by VSCode by default. And the current supported VSCode versions still include some that need the flag.

[jasonlyu123]

Sorry, never mind, I misunderstood what you meant.

I think we also don't need to add the flag when the svelte.language-server.runtime config is set. We won't know the version of the custom node.js version, and users can still use the svelte.language-server.runtime-args config to add it.

[craig-jennings]

@jasonlyu123 I believe I addressed your comments in the latest commit. When you have a sec, could you take another look?

[jasonlyu123]

Found that Node.js has an API for checking if --experimental-strip-types is enabled. Can add a check to the configLoader to load ts files if it's enabled.

const loadSvelteConfigTs = 'typescript' in process.features && !!process.features.typescript

[craig-jennings]

@jasonlyu123 Not quite sure I'm following where you're wanting loadSvelteConfigTs check to go, so feel free to update this PR as you see fit.