Refactor and clean up mode syntax

[riaqn]

This PR imports the latest changes from the compiler, and does some clean-up:

• The parser.mly changes from Refactor new mode syntax oxcaml#3522 is adapted to parser-extended
• Some other minor changes from the compiler parser are also adapted. But I believe some are still missing.
• Modes on modules are parsed but not supported. (the type checking PR is at Support modules of non-legacy modes oxcaml#3759 , we need to support this syntax once that PR is merged).

Currently it has a few issues:

• comments' relative locations are not preserved
• indentation are messy

[riaqn]

I've reviewed @dvulakh 's commits until "fix handling of parser edge-case"

[dvulakh]

I'm happy to merge this in its current state. I haven't been able to figure out why the MacOS CI is broken, but am in favor of merging anyways and fixing later if we end up internally blocked on this PR.