Generate .d.ts bundle from source

[Rich-Harris]

I nerd-sniped myself in #9895 (comment) — I've created a small library, dts-buddy (couldn't think of a better name), for generating .d.ts bundles.

It's currently missing two important features:

• only exporting the things you actually want to export, instead of everything
• sourcemaps

Also, virtual modules like __sveltekit/environment are currently a TODO. We also need to

• update the doc generation logic

Once those things are fixed, I think this gives us the best of all possible worlds — everything is derived from source (no more duplicating function definitions inside the hand-authored .d.ts files), and 'go to definition' will finally work properly. We also get to delete a bunch of stuff.

Please don't delete this checklist! Before submitting the PR, please make sure you do the following:

• It's really useful if your PR references an issue where it is discussed ahead of time. In many cases, features are absent for a reason. For large changes, please create an RFC: https://github.com/sveltejs/rfcs
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• Ideally, include a test that fails without this PR but passes with it.

Tests

• Run the tests with pnpm test and lint the project with pnpm lint and pnpm check

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[Rich-Harris]

One compromise — had to expose __sveltekit/paths and __sveltekit/environment. Couldn't come up with a better solution that wouldn't have involved a lot more work.

That said, I realised just now that we're currently messing up by referencing these modules anyway, and need to come up with a different solution. Both of them indirectly reference the app version, which defeats the goal of maintaining hash stability between deployments. I therefore think we need to rethink how that's implemented, but that's out of scope of this PR so I'm going to punt on it for now.

[elliott-with-the-longest-name-on-github]

l.g.t.m.
a o e o 
u d n m 
d l d e 
e y i n 
d   n t 
    g a 
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      i 
      l 
      y 

[elliott-with-the-longest-name-on-github]

TIL -- I was searching for this a while ago and couldn't find it!

[benmccann]

lgtm. for those who haven't been following along, the motivation for this PR is explained in #9883 (comment)