Coming from discussion on #25936 and
https://github.com/zed-industries/zed/pull/19756 .
This PR introduces the KernelSession abstraction and adds basic notebook
cell execution inside NotebookEditor.

The following provides a base starter for the development on Notebooks.

What this PR includes today:

Release Notes:

- KernelSession trait extracted. Both REPL and NotebookEditor now share
the same routing mechanism.
- NotebookEditor can launch kernels, execute code cells, and display
outputs.
- Basic cell operations: run current cell, run all, move up/down, add
code/markdown blocks. Keybindings follow Jupyter defaults.


Next Steps:

- [x] Editing support for markdown and code blocks.
- Buffer integration instead of temporary cell text.
- [x] Proper focus behavior when executing or adding cells.
- Kernel control UI.

A little far fetched steps:

- Vim Support
- Cell Handling Improvement and other convenient features and design
from other editors
- Ability to have better parsing for AI Support. 


I have attached a video of showcasing some of the features:


https://github.com/user-attachments/assets/37e6f3e5-2022-45f0-a73d-2dd01ebc2932