- Introducing NotebookMode state and handlers (EnterEditMode,
EnterCommandMode, RunAndAdvance).
- Wire up UI to switch modes,focus editors appropriately, and advance
selection while in command mode.
- Update default and vim keymaps to use the new bindings (shift-enter
runs+advances, escape enters command mode,
and enter/up/down work in command mode).
- Reveal and Focus the new Cell when inserted 

Before you mark this PR as ready for review, make sure that you have:
- [x] Added a solid test coverage and/or screenshots from doing manual
testing
- [x] Done a self-review taking into account security and performance
aspects
- [x] Aligned any UI changes with the [UI
checklist](https://github.com/zed-industries/zed/blob/main/CONTRIBUTING.md#uiux-checklist)

Release Notes:

- N/A