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/ui/src/components/keybinding_hint.rs

@@ -1,7 +1,6 @@
 use crate::KeyBinding;
 use crate::{h_flex, prelude::*};
 use gpui::{AnyElement, App, BoxShadow, FontStyle, Hsla, IntoElement, Window, point};
-use smallvec::smallvec;
 use theme::Appearance;
 
 /// Represents a hint for a keybinding, optionally with a prefix and suffix.
@@ -193,7 +192,7 @@ impl RenderOnce for KeybindingHint {
                     .border_1()
                     .border_color(border_color)
                     .bg(bg_color)
-                    .shadow(smallvec![BoxShadow {
+                    .shadow(vec![BoxShadow {
                         color: shadow_color,
                         offset: point(px(0.), px(1.)),
                         blur_radius: px(0.),