[WIP/POC] add diff completion item

[akoreman]

Issue #, if available: NA

Description of changes: Trying to see whether it could make sense to add diff completion items which, when inserted, apply a diff to the file which could affect more than just the current cursor position.

Screen.Recording.2025-03-05.at.21.12.06.mov

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Pull Request Checklist:

• No backwards incompatible changes were made to Ace's public interface which is defined through the main typings file (ace.d.ts) and its references:
  • https://github.com/ajaxorg/ace/blob/master/ace.d.ts
  • https://github.com/ajaxorg/ace/blob/master/ace-modes.d.ts
  • https://github.com/ajaxorg/ace/blob/master/ace-extensions.d.ts