The new command's display name is ".NET Interactive: Create new blank notebook" for two reasons:

1. All of our other command names start with ".NET Interactive:" to remove any ambiguity and I thought that should continue.
2. The Python command to create a new Jupyter notebook is "Create New Blank Jupyter Notebook".

I've added a keybinding of Ctrl+Alt+Shift+N to invoke this. It's extremely verbose, but I don't want to steal any existing keybinding, and unfortunately that means we can't use the chord Ctrl+N, D because that causes the default Ctrl+N to be unbound because it's waiting for a chord to continue, and that's bad. An alternative is to not have a default keybinding and let users add their own that simply invokes the command dotnet-interactive.newNotebook. I'm open to either approach.

As for changes to code, there were really only 3 of note:

1. To create a new untitled notebook, I avoided calling workbench.action.files.newUntitledFile { viewType: 'dotnet-interactive' } because that creates a new notebook with the name Untitled-1 (note, no file extension) and when the user hits Ctrl+S, the save dialog defaults to the .txt extension, and that can cause a whole bunch of problems because the file format is chosen depending on the extension, and if the user chooses .txt (or .my-notebook, etc.) we have no idea how to serialize the notebook. Instead, I manually create a URI with the .dib file extension pre-applied. This means when the user finally saves their notebook, it'll have the correct extension. It's still possible for them to manually override this, but that's on them; if they're mangling the file extensions we can't help them. To enable multiple unsaved notebooks, I look through the existing notebooks until I can find a numeric suffix that's unused, e.g., Untitled-9.dib.

2. Add the method ClientMapper.reassociateClient. This is necessary because the notebook's runtime is associated with the notebook's URI. In normal scenarios, this will be something like C:\Users\brettfo\Desktop\my-notebook.dib. In the case of a new unsaved notebook, the URI is Untitled-1.dib. Initially this is fine, but as soon as the user saves the notebook, the URI will suddenly be a file path like above and since we don't want them to loose their current runtime values and leave around a stale kernel, the reassociate method does exactly that: reassociates the kernel process with the new URI.

3. VS Code will call openNotebook when creating a new notebook viewer, but if it's an unsaved notebook created through the new command, nothing exists on disk. The fix for this was simple; if it's a new unsaved file, short-circuit everything and create an empty notebook.