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/styles/elevation.rs

@@ -1,7 +1,6 @@
 use std::fmt::{self, Display, Formatter};
 
 use gpui::{App, BoxShadow, Hsla, hsla, point, px};
-use smallvec::{SmallVec, smallvec};
 use theme::{ActiveTheme, Appearance};
 
 /// Today, elevation is primarily used to add shadows to elements, and set the correct background for elements like buttons.
@@ -40,14 +39,14 @@ impl Display for ElevationIndex {
 
 impl ElevationIndex {
     /// Returns an appropriate shadow for the given elevation index.
-    pub fn shadow(self, cx: &App) -> SmallVec<[BoxShadow; 2]> {
+    pub fn shadow(self, cx: &App) -> Vec<BoxShadow> {
         let is_light = cx.theme().appearance() == Appearance::Light;
 
         match self {
-            ElevationIndex::Surface => smallvec![],
-            ElevationIndex::EditorSurface => smallvec![],
+            ElevationIndex::Surface => vec![],
+            ElevationIndex::EditorSurface => vec![],
 
-            ElevationIndex::ElevatedSurface => smallvec![
+            ElevationIndex::ElevatedSurface => vec![
                 BoxShadow {
                     color: hsla(0., 0., 0., 0.12),
                     offset: point(px(0.), px(2.)),
@@ -59,10 +58,10 @@ impl ElevationIndex {
                     offset: point(px(1.), px(1.)),
                     blur_radius: px(0.),
                     spread_radius: px(0.),
-                }
+                },
             ],
 
-            ElevationIndex::ModalSurface => smallvec![
+            ElevationIndex::ModalSurface => vec![
                 BoxShadow {
                     color: hsla(0., 0., 0., if is_light { 0.06 } else { 0.12 }),
                     offset: point(px(0.), px(2.)),
@@ -89,7 +88,7 @@ impl ElevationIndex {
                 },
             ],
 
-            _ => smallvec![],
+            _ => vec![],
         }
     }