chore: Build and publish v4 `lib/` and `themes/` folders for backwards compatibility while users transition to v5. (#2667)

* Build and publish v4 lib/ and themes/ folders for backwards compatibility while users transition to v5.

- add a script to build the legacy v4 lib/ and themes/ folders
- append deprecation console warnings to the v4 JS files during the v4 build
- add a script to clean v4 build outputs
- hook the v4 build/clean scripts into the release.sh script
- test the v4 build and clean in CI

* fix: update repository field format in package.json

---------

Co-authored-by: Luffy <[email protected]>