Sequential navigation in edit history (#13418)

* First commit - sequential navigation in edit history

* Fix lint errors

* Handle undefined history entries

* Added comments and some additional edge case handling

* Update code review comments

* Put a max navigation frame capture limit and control overflow once reached. Fix all bugs found in unit testing.

* Fix indentation issues

* Change spaces to tabs for indentation

* Fix mixed tabs and spaces

* Convert spaces to tab

* resolve mixed tabs and spaces

* Initialize variables

* Handle external file changes and discard stale frames

* Additional checks to validate the navigation frames

* Address review comments

src/editor/Editor.js

@@ -1017,6 +1017,9 @@ define(function (require, exports, module) {
         this._codeMirror.on("cursorActivity", function (instance) {
             self.trigger("cursorActivity", self);
         });
+        this._codeMirror.on("beforeSelectionChange", function (instance, selectionObj) {
+            self.trigger("beforeSelectionChange", selectionObj);
+        });
         this._codeMirror.on("scroll", function (instance) {
             // If this editor is visible, close all dropdowns on scroll.
             // (We don't want to do this if we're just scrolling in a non-visible editor