Introduce SourceDocument for text edit support

[kazcw]

Pull Request Description

Introduce SourceDocument, a reactive source code representation that can be synced from a MutableModule. SourceDocument replaces various logic for tracking source code and spans--most importantly, ReactiveModule. There is no longer any reactively-tracked Module, per-se: Changes to the MutableModule attached to the synchronized ydoc are pushed as Y.Js events to the SourceDocument and GraphDb, which are reactively tracked. This avoids a problem in the upcoming text-synchronization (next PR) that was caused by a reactive back channel bypassing the GraphDb and resulting in observation of inconsistent states.

Stacked on #8956. Part of #8238.

Important Notes

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.

[farmaazon]

Please add some docs about this class, it will help to read the code.