devtools: Allow highlighting elements from the inspector (#35822)

This change connects the `HighlighterActor` from the devtools with the
document, which will draw a blue rectangle over any highlighted dom
node.



https://github.com/user-attachments/assets/571b2dab-497f-4102-9e55-517cdcc040ba




---
<!-- Thank you for contributing to Servo! Please replace each `[ ]` by
`[X]` when the step is complete, and replace `___` with appropriate
data: -->
- [X] `./mach build -d` does not report any errors
- [X] `./mach test-tidy` does not report any errors
- [X] These changes do not require tests because we don't have devtools
tests

Signed-off-by: Simon Wülker <simon.wuelker@arcor.de>

components/shared/script_layout/lib.rs

@@ -428,6 +428,8 @@ pub struct ReflowRequest {
     pub node_to_image_animation_map: FxHashMap<OpaqueNode, ImageAnimationState>,
     /// The theme for the window
     pub theme: PrefersColorScheme,
+    /// The node highlighted by the devtools, if any
+    pub highlighted_dom_node: Option<OpaqueNode>,
 }
 
 /// A pending restyle.