Generate Lit 3.0 pre-release API docs under v3 subdirectory (#1113)

* Copy over all unversioned content into a versioned directory

The only file changed in the move is the root `docs.json` file. Instead
a `v2.json` file was added.

* Add initial build-unversioned-docs script.

This copies over and adds the correct permalinks to the selected
version content. Currently moving `v2` -> `unversioned`.

* Fix wireit so npm run dev doesnt spin infinitely.

* Add rel=canonical link from latestVersion to unversioned page.

* Make all authored cross links versioned.

Unversioned links will be generated automatically by the generated
unversioned pages.

* Add fixUnversionedCrossLinks so unversioned cross-linking work.

This works by piping all the content through a function that will
fix cross links on unversioned pages.

* Add integration tests checking added features.

* unblock failing link by adding it to the known good list.

* Fix api shortcode such that it uses a versioned URL

The versioned URL is stripped for unversioned URLs by our tooling.

Add a test to confirm this behavior.

* Apply code review feedback. Thank you!

* Code review feedback from Justin

 - Replaced sync fs with async fs.
 - Made lightweight YAML parser more robust using regex.

* Add undiscoverable v3 generated documentation.

Currently the Lit 3 commit being used is
d04a3e30eb3ae3520fb0ac163fb5ddbbf6030620 because
lit/lit#3894 fixes building Lit 3.

API docs are visible at /docs/api/v3/

* Explicitly bump node heap size.

This will hopefully avoid the Github Actions failures.

* Add types/codemirror to fix TS 5 error.

Thank you Augustine! Re-ran generator with no content changes.

* Remove unneeded dependency

Co-authored-by: Augustine Kim <augustinekim@google.com>

* Code review feedback with Wireit env.

* Fix formatting & filter out v3/api/index.html page

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Co-authored-by: Augustine Kim <augustinekim@google.com>

Author: AndrewJakubowicz