Extension refactor (#20305)

This contains the main changes to the extensions crate from #20049. The
primary goal here is removing dependencies that we can't include on the
remote.


Release Notes:

- N/A

---------

Co-authored-by: Mikayla <mikayla@zed.dev>
Co-authored-by: Marshall Bowers <elliott.codes@gmail.com>
Co-authored-by: Marshall <marshall@zed.dev>

crates/workspace/src/pane_group.rs

@@ -357,7 +357,7 @@ impl PaneAxis {
 
     pub fn load(axis: Axis, members: Vec<Member>, flexes: Option<Vec<f32>>) -> Self {
         let flexes = flexes.unwrap_or_else(|| vec![1.; members.len()]);
-        debug_assert!(members.len() == flexes.len());
+        // debug_assert!(members.len() == flexes.len());
 
         let flexes = Arc::new(Mutex::new(flexes));
         let bounding_boxes = Arc::new(Mutex::new(vec![None; members.len()]));