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/text_system.rs

@@ -583,7 +583,7 @@ impl DerefMut for LineWrapperHandle {
 
 /// The degree of blackness or stroke thickness of a font. This value ranges from 100.0 to 900.0,
 /// with 400.0 as normal.
-#[derive(Clone, Copy, Debug, PartialEq, PartialOrd, Deserialize, Serialize, JsonSchema)]
+#[derive(Clone, Copy, Debug, PartialEq, PartialOrd, Serialize, Deserialize, JsonSchema)]
 pub struct FontWeight(pub f32);
 
 impl Default for FontWeight {
@@ -636,7 +636,7 @@ impl FontWeight {
 }
 
 /// Allows italic or oblique faces to be selected.
-#[derive(Clone, Copy, Eq, PartialEq, Debug, Hash, Default)]
+#[derive(Clone, Copy, Eq, PartialEq, Debug, Hash, Default, Serialize, Deserialize, JsonSchema)]
 pub enum FontStyle {
     /// A face that is neither italic not obliqued.
     #[default]