Syntactic synchronization

[kazcw]

Pull Request Description

Synchronize Y.Js clients by AST.

Implements #8237.

Important Notes

Reviewers: The first commit is mostly organizing things, the smaller commit is more interesting.

Parent-pointer consistency checks are not implemented yet. That will be important for multi-client, but it should be impossible to create inconsistent parents with only one user editing.

Checklist

Please ensure that the following checklist has been satisfied before submitting the PR:

• The documentation has been updated, if necessary.
• Screenshots/screencasts have been attached, if there are any visual changes. For interactive or animated visual changes, a screencast is preferred.
• All code follows the
  Scala,
  Java,
  and
  Rust
  style guides. In case you are using a language not listed above, follow the Rust style guide.
• All code has been tested:
  • Unit tests have been written where possible.
  • If GUI codebase was changed, the GUI was tested when built using ./run ide build.

[kazcw]

I still have to get e2e tests working with the new model, but aside from the mock code this is ready for review.

[kazcw]

Integrated into #8893 (all review not addressed here has now been addressed there).