Implement new Enso documentation parser; remove old Scala Enso parser.
Performance: Total time parsing documentation is now ~2ms.
# Important Notes
- Doc parsing is now done only in the frontend.
- Some engine tests had never been switched to the new parser. We should investigate tests that don't pass after the switch: #5894.
- The option to run the old searcher has been removed, as it is obsolete and was already broken before this (see #5909).
- Some interfaces used only by the old searcher have been removed.
