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/feature_flags/src/feature_flags.rs

@@ -59,6 +59,12 @@ impl FeatureFlag for ZedPro {
     const NAME: &'static str = "zed-pro";
 }
 
+pub struct NotebookFeatureFlag;
+
+impl FeatureFlag for NotebookFeatureFlag {
+    const NAME: &'static str = "notebooks";
+}
+
 pub struct AutoCommand {}
 impl FeatureFlag for AutoCommand {
     const NAME: &'static str = "auto-command";