feat: resolve Svelte components using TS from exports map

[dummdidumm]

This change allows people to write export maps using only a svelte condition (and no types condition) and still have the types for their components resolved (i.e. the import is found) as long as they use TypeScript (i.e. have lang="ts" attribute) inside it. This should help people using monorepo setups with strong typings and not wanting to provide d.ts files alongside.

This is achieved doing three adjustments:

• add customConditions: ['svelte'] to the compiler options, so that TypeScript's resolution algorithm takes it into account
• ~ensure that Svelte files have a module kind of ESM, so that TypeScript's resolution algorithm goes into the right branches~ no longer needed since the switch to using .d.svelte.ts as the "virtual file check" entry point
• deal with .d.svelte.ts files in the context of an exports map, because that's what TypeScript will try to resolve this to in the end

This is also related to #1056 insofar that we align with TypeScript for this new capability: We don't resolve the file if it's a component not using TypeScript (i.e. not having the lang="ts" tag), similar to how TypeScript does not resolve .js files within node_modules

As a side effect, this is a major version bump for language-tools and the typescript plugin, and consequently for svelte-check/vs code extension

[dummdidumm]

Mhm not sure why this fails - works locally for me (Windows)

[jasonlyu123]

I didn't see the package.json with exportMap. Maybe it's ignored because of gitignore? About .d.svelte.ts , this isn't exclusive to export maps import. TypeScript would also try this with relative import. Not sure if anyone uses it to override the type definition of the svelte file. This will change the resolved module from the .d.svelte.ts to the actual svelte file.

[dummdidumm]

Doh, you're right, the node modules folder isn't checked in. Good point about the files - theoretically can happen for svelte.ts, too. I'll look into it

[dummdidumm]

I realized that since 5.0, TypeScript will check for d.svelte.ts first before checking .svelte.ts etc - which we can take to our advantage to do the following:

• be less exposed to the danger of clashing with a real .svelte.ts file (in Svelte 5 this will be more common)
• when doing import Foo from './foo.svelte, foo.svelte will now take precedence over a sibling foo.svelte.ts file. You can import that sibling by doing import { foo } from './foo.svelte.js'. It's definitely discouraged to have files with the same name, but it's possible now. Strictly speaking this is a breaking change because the order is different now

[dummdidumm]

Having second thoughts about this - should we really do this? It could encourage bad patterns. On the other hand we could also embrace it - I'm not sure. This needs discussion with other maintainers.

Implementation-wise I noticed that the TS plugin does throw an error: Module 'package' was resolved to '[redacted]/node_modules/package/foo.svelte', but '--allowArbitraryExtensions' is not set.ts(6263) - not sure if there's a way to get around this, which would also be a problem for this PR.

I'm gonna split out the other changes to land them and put this into draft.