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/gpui/src/elements/list.rs

@@ -9,14 +9,13 @@
 
 use crate::{
     AnyElement, App, AvailableSpace, Bounds, ContentMask, DispatchPhase, Edges, Element, EntityId,
-    FocusHandle, GlobalElementId, Hitbox, IntoElement, Pixels, Point, ScrollWheelEvent, Size,
-    Style, StyleRefinement, Styled, Window, point, px, size,
+    FocusHandle, GlobalElementId, Hitbox, InspectorElementId, IntoElement, Overflow, Pixels, Point,
+    ScrollWheelEvent, Size, Style, StyleRefinement, Styled, Window, point, px, size,
 };
 use collections::VecDeque;
 use refineable::Refineable as _;
 use std::{cell::RefCell, ops::Range, rc::Rc};
 use sum_tree::{Bias, SumTree};
-use taffy::style::Overflow;
 
 /// Construct a new list element
 pub fn list(state: ListState) -> List {
@@ -820,9 +819,14 @@ impl Element for List {
         None
     }
 
+    fn source_location(&self) -> Option<&'static core::panic::Location<'static>> {
+        None
+    }
+
     fn request_layout(
         &mut self,
         _id: Option<&GlobalElementId>,
+        _inspector_id: Option<&InspectorElementId>,
         window: &mut Window,
         cx: &mut App,
     ) -> (crate::LayoutId, Self::RequestLayoutState) {
@@ -890,6 +894,7 @@ impl Element for List {
     fn prepaint(
         &mut self,
         _id: Option<&GlobalElementId>,
+        _inspector_id: Option<&InspectorElementId>,
         bounds: Bounds<Pixels>,
         _: &mut Self::RequestLayoutState,
         window: &mut Window,
@@ -938,6 +943,7 @@ impl Element for List {
     fn paint(
         &mut self,
         _id: Option<&GlobalElementId>,
+        _inspector_id: Option<&InspectorElementId>,
         bounds: Bounds<crate::Pixels>,
         _: &mut Self::RequestLayoutState,
         prepaint: &mut Self::PrepaintState,