Support Untitled files and automatic *.Notebook.ps1 files

[TylerLeonhardt]

PR Summary

Fixes #2866
Fixes #2901
Fixes #2865

This enables the following features:

• Untitled file support - now you can open untitled PowerShell files in Notebook mode, when you save the untitled notebook the experience is what you expect (saves to ps1 and opens that ps1 in notebook mode)
• Automatic Notebooks - if you create a file with *.Notebook.ps1, it will automatically open in Notebook mode. In order to do this, I needed to register 2 different viewTypes with the same underlying implementation. It's a little clunky from the dev experience (I'll probably open an issue) but the UX feels nice. You can still go back to text editor mode from a *.Notebook.ps1 if you want

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[TylerLeonhardt]

in a few insider builds we can remove the onNotebookEditor ones as that will be deprecated.