TextEditorDiffInformation API proposal (#233896)

* WIP - initial implementation

* Introduce the diff model service

* Remove code that is not needed

* Handle DiffEditor

* Performance optimization

* Refactor code

* More cleanup (V1)

* More cleanup (V2)

* More cleanup (V2.1)

* Pull request feedback

* Remove debugging statements

* Update mock proxy to fix tests

* Add proposed api check

extensions/git/package.json

@@ -34,6 +34,7 @@
     "scmValidation",
     "tabInputMultiDiff",
     "tabInputTextMerge",
+    "textEditorDiffInformation",
     "timeline"
   ],
   "categories": [