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/inspector_ui/src/inspector_ui.rs

@@ -0,0 +1,24 @@
+#[cfg(debug_assertions)]
+mod div_inspector;
+#[cfg(debug_assertions)]
+mod inspector;
+
+#[cfg(debug_assertions)]
+pub use inspector::init;
+
+#[cfg(not(debug_assertions))]
+pub fn init(_app_state: std::sync::Arc<workspace::AppState>, cx: &mut gpui::App) {
+    use std::any::TypeId;
+    use workspace::notifications::NotifyResultExt as _;
+
+    cx.on_action(|_: &zed_actions::dev::ToggleInspector, cx| {
+        Err::<(), anyhow::Error>(anyhow::anyhow!(
+            "dev::ToggleInspector is only available in debug builds"
+        ))
+        .notify_app_err(cx);
+    });
+
+    command_palette_hooks::CommandPaletteFilter::update_global(cx, |filter, _cx| {
+        filter.hide_action_types(&[TypeId::of::<zed_actions::dev::ToggleInspector>()]);
+    });
+}