remove custom .ipynb handling and update apis

[brettfo]

Remove all custom Jupyter/.ipynb handling. The default new notebook is still .ipynb until microsoft/vscode#121974 is fixed, but then we'll default to .dib.

Scenarios to validate in Insiders:

1. Double-click existing .NET .ipynb; our controller should be selected and cell languages should be correct. Our controller is properly selected based on our specified affinity, but due to onDidChangeNotebookAssociation not fired on initial notebook open microsoft/vscode#121904 the cell languages aren't yet correct. There is currently no workaround.
2. Double-click existing .dib; everything should work as before. This is good.
3. New Jupyter notebook, select one of our languages. Our kernel controller should be selected. Our controller is properly selected, but again, due to onDidChangeNotebookAssociation not fired on initial notebook open microsoft/vscode#121904 the cell languages aren't yet correct.

[colombod]

in my environment I don't have python extension so our controller is the only option
So far:

1. open dib works
2. new blank notebook using the command Jupyter : create new blank notebook selects our controller but languages are wrong
3. open ipynb our controller is selected but languages are wrong
4. new blank notebook using the command .NET Interactive : create new blank notebook selects our controller but languages are wrong
5. Can't close a dib with pending changes
6. Can't set a notebook to trusted

[colombod]

Our notebook handling setting medatada on cells when working with dib files makes the document dirty and that causes the infinite loop of reopen when closing a dirty document even discarding the edits, the document seems to be modified and vscode will reopen