Coexistence of native notebook and Vim

[rebornix]

This issue will be used as a roadmap for supporting Vim seamlessly in the native notebook editor in VS Code. The native notebook consists of text editors, in which Vim can be enabled. However there are conflicts between Jupyter style commands and Vim's builtin ones. The Vim extension currently is not aware of the notebook concept thus it usually doesn't escape the cell editor.

Known issues:

• jk on cell boundary (it should move to near cell)
• alt+delete to clear output doesn't work