Port custom editor changes to release branch (#12222)

* Implement the changes necessary to use the new custom editor (#12188)

* Turn on custom editor again

* Fix copy problem

* Fix copy problem again

* Get undo/redo to work

* Some fixes for synching more than one editor

* Fix untitled. Fix timeouts

* fix command manager to not be used. Not necessary with temp path.
fix delete/insert commands to undo properly

* Fix functional test

* Add experiment

* Since package json is enabling proposed api, turn of notebooks with experiment

* Turn off proposed api

* Upgrade VS code to 1.45 so can use new api outside of insiders

* Update package.json dynamically

* Fix merge code to handle arrays

* Fix unit tests. Backup happens at the provider level now

* Fix hygiene

* Fix untitled to work for old provider too

* Some feedback and attempt to fix URI problem on linux/mac

* Review feedback

* Update changelog

* Remove news entry

Author: rchiodo

CHANGELOG.md

@@ -1,6 +1,6 @@
 # Changelog
 
-## 2020.6.0-rc (8 June 2020)
+## 2020.6.0-rc (15 June 2020)
 
 ### Enhancements
 
@@ -10,6 +10,8 @@
    ([#11057](https://github.com/Microsoft/vscode-python/issues/11057))
 1. Preliminary support using other languages for the kernel.
    ([#11919](https://github.com/Microsoft/vscode-python/issues/11919))
+1. Enable the use of the custom editor for native notebooks.
+   ([#10744](https://github.com/Microsoft/vscode-python/issues/10744))   
 
 ### Fixes