Create a concat doc that listens to LSP events to modify itself

[rchiodo]

Replace the current notebook concat document with an implementation that can be shared with pylance.

[rchiodo]

Start in jupyter extension. Overlapping period might switch to npm module.

[rchiodo]

This is essentially complete. Implementation resides here:
https://github.com/microsoft/vscode-jupyter-lsp-middleware/blob/88a75b1b17118604ba30c30f98fbf113b0f540a9/src/notebookConcatDocument.ts#L36

It will likely change as I fix bugs and such but should be portable now.

[rchiodo]

Verification steps:

• Open a notebook
• Verify semantic colorization happens
• Verify hover works
• Edit cells/Move cells/Delete cells
• Verify diagnostics are as expected (as if the contents of all python cells were concatenated together)