Test undo stack survival on buffer dispose

[alexdima]

Testing - #7169

• anyOS @sandy081
• anyOS @alexr00

Complexity: 3

---

Please check:

• open a file
• make edits to it
• save the file
• close the file
• open the file again
• undo/redo should work

Extra info / please check:

• this works only for file and vscode-remote text models.
• if the undo/redo buffer is very large, a prompt is shown to confirm that storing the memory is OK
• if the file is edited on disk (while not opened in vscode), when the file is opened the undo/redo buffer is dropped without any prompt by using a content hash.