JS: Support TypeScript in HTML/Vue files

[asgerf]

Adds support for TypeScript code embedded in HTML and Vue files.

There were a number of complications to overcome here:

• HTML extraction happens in the multi-threaded extraction phase, after the TypeScript compiler has been shut down.
  • I moved HTML extraction first, and made it spit out .ts snippets in the virtual source root, to be extracted during TypeScript extraction.
• TypeScript must now be able to find files in the virtual source root, and resolve imports in both directions.
• Source locations for these virtual files must be mapped back to the original file.
• QLTest uses the old extractor entry point, which doesn't have a notion of a source root or scratch dir, which is needed for testing features that rely on virtual source root. As a short-term solution, it now makes up reasonable values for source root and scratch dir on its own.

Evaluation (bundled with #3833 and #3834) looks uneventful.

Fixes #3730

[erik-krogh]

Wow, that is a mouthful.

I'll take a final look later, just a small comment for now.

[erik-krogh]

The many small commits really helped understand this PR 👍

[asgerf]

Thanks for going through it all @erik-krogh! 👍