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/refineable/src/refineable.rs

@@ -1,7 +1,7 @@
 pub use derive_refineable::Refineable;
 
 pub trait Refineable: Clone {
-    type Refinement: Refineable<Refinement = Self::Refinement> + Default;
+    type Refinement: Refineable<Refinement = Self::Refinement> + IsEmpty + Default;
 
     fn refine(&mut self, refinement: &Self::Refinement);
     fn refined(self, refinement: Self::Refinement) -> Self;
@@ -13,6 +13,11 @@ pub trait Refineable: Clone {
     }
 }
 
+pub trait IsEmpty {
+    /// When `true`, indicates that use applying this refinement does nothing.
+    fn is_empty(&self) -> bool;
+}
+
 pub struct Cascade<S: Refineable>(Vec<Option<S::Refinement>>);
 
 impl<S: Refineable + Default> Default for Cascade<S> {