Initial Notebook UI structure (#19756)

This is the start of a notebook UI for Zed. 

`🔔 Note: This won't be useable yet when it is merged! Read below. 🔔`

This is going to be behind a feature flag so that we can merge this
initial PR and then make follow up PRs. Release notes will be produced
in a future PR.

Minimum checklist for merging this:

* [x] All functionality behind the `notebooks` feature flag (with env
var opt out)
* [x] Open notebook files in the workspace
* [x] Remove the "Open Notebook" button from title bar
* [x] Incorporate text style refinements for cell editors
* [x] Rely on `nbformat` crate for parsing the notebook into our
in-memory format
* [x] Move notebook to a `gpui::List`
* [x] Hook up output rendering


Release Notes:

- N/A

---------

Co-authored-by: Nate Butler <iamnbutler@gmail.com>
Co-authored-by: Thorsten Ball <mrnugget@gmail.com>

crates/zed/src/main.rs

@@ -418,6 +418,7 @@ fn main() {
         app_state.languages.set_theme(cx.theme().clone());
         editor::init(cx);
         image_viewer::init(cx);
+        repl::notebook::init(cx);
         diagnostics::init(cx);
 
         audio::init(Assets, cx);