Add initial element inspector for Zed development (#31315)

Open inspector with `dev: toggle inspector` from command palette or
`cmd-alt-i` on mac or `ctrl-alt-i` on linux.

https://github.com/user-attachments/assets/54c43034-d40b-414e-ba9b-190bed2e6d2f

* Picking of elements via the mouse, with scroll wheel to inspect
occluded elements.

* Temporary manipulation of the selected element.

* Layout info and JSON-based style manipulation for `Div`.

* Navigation to code that constructed the element.

Big thanks to @as-cii and @maxdeviant for sorting out how to implement
the core of an inspector.

Release Notes:

- N/A

---------

Co-authored-by: Antonio Scandurra <me@as-cii.com>
Co-authored-by: Marshall Bowers <git@maxdeviant.com>
Co-authored-by: Federico Dionisi <code@fdionisi.me>

crates/workspace/src/pane_group.rs

@@ -1113,9 +1113,14 @@ mod element {
             Some(self.basis.into())
         }
 
+        fn source_location(&self) -> Option<&'static core::panic::Location<'static>> {
+            None
+        }
+
         fn request_layout(
             &mut self,
             _global_id: Option<&GlobalElementId>,
+            _inspector_id: Option<&gpui::InspectorElementId>,
             window: &mut Window,
             cx: &mut App,
         ) -> (gpui::LayoutId, Self::RequestLayoutState) {
@@ -1132,6 +1137,7 @@ mod element {
         fn prepaint(
             &mut self,
             global_id: Option<&GlobalElementId>,
+            _inspector_id: Option<&gpui::InspectorElementId>,
             bounds: Bounds<Pixels>,
             _state: &mut Self::RequestLayoutState,
             window: &mut Window,
@@ -1224,6 +1230,7 @@ mod element {
         fn paint(
             &mut self,
             _id: Option<&GlobalElementId>,
+            _inspector_id: Option<&gpui::InspectorElementId>,
             bounds: gpui::Bounds<ui::prelude::Pixels>,
             _: &mut Self::RequestLayoutState,
             layout: &mut Self::PrepaintState,

crates/workspace/src/workspace.rs

@@ -7277,7 +7277,7 @@ pub fn client_side_decorations(
                         .when(!tiling.left, |div| div.border_l(BORDER_SIZE))
                         .when(!tiling.right, |div| div.border_r(BORDER_SIZE))
                         .when(!tiling.is_tiled(), |div| {
-                            div.shadow(smallvec::smallvec![gpui::BoxShadow {
+                            div.shadow(vec![gpui::BoxShadow {
                                 color: Hsla {
                                     h: 0.,
                                     s: 0.,