EDI-1607 Implement MArray equivalent of trackArray

[mar-h4]

• Implemented event handler class and moved it to a new marray-utils.ts file;
• Implemented a MArray.track method that adds onCreate and onRemove listeners to the MArray class, and returns functions to remove the listeners;
• Implemented private remove and insert methods that wrap the direct tree modification methods and run the onCreate and onRemove handlers accordingly;
• Replaced all tu.insert and tu.remove usages with the newly created ones in all MArray methods that modify the string, as well as in the update method;
• Added tests to confirm behavior;
• Bonus: Fixed the MArray.pop implementation to handle empty array case (should return undefined by spec).

Task: EDI-1607

[spion]

Other than the minor nit, looks good