* Implement `clone_on_split` to allow splitting a notebook into another
pane
* Switched to `tab_content` in `impl Item for NotebookEditor` to show
both the notebook name and an icon
* Added placeholder methods and TODOs for future work, such as saving,
reloading, and search functionality within the notebook editor.
* Started moving more core `Model` bits into `NotebookItem`, including
pulling the language of the notebook (which affects every code cell)
* Loaded notebook asynchronously using `fs`
Release Notes:
- N/A
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Co-authored-by: Mikayla <mikayla@zed.dev>
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