This PR removes the old typescript-based theme system, and it's related
functions.

TODO:
- [x] Create standalone repo of old theme system in case it is needed.
  - https://github.com/zed-industries/zed1-styletrees
- [x] Remove `styles`
- [x] Remove licenses generating code related to `styles`
- [x] Remove docs related to old theme system
- [x] Add license generation to new theme system (#3917)
- [x] Check as best possible that all the places we were accessing
`styles` are removed or updated.

Note: For now, the JSON files in `assets/themes` will remain until we
are finished with the Zed 1 theme importer. We should be able to remove
those in the next few weeks.

Release Notes:

- N/A