* new API option and flag
* show the notebook as a repl
* handle any notebook type, dispose all model refs
* open notebook doc as scratchpad
* delay loading the model for untitled notebooks until shown
* add initial content consistently for untitled
* simplify utitled notebook creation
* recover open untitled notebook as dirty behavior
For #147248
When stepping through the liveshare code, we figured out they do not appear to be using `save`, `saveAs`, or `backup` (they return empty results for these)
This PR removes this part of the proposal so we can track just the parts of the api that they are using
Aaron Munger
Don Jayamanne
Peng Lyu
Matt Bierner
Rob Lourens
Logan Ramos