Add SimulateColorScheme command and script thread handling (#36253)

Implements Steps 2-3 of #35867:
- Adds `SimulateColorScheme` to `DevtoolScriptControlMsg` for light/dark
mode simulation.
- Handles it in `ScriptThread` with `handle_theme_change` to toggle
themes.

Testing: This PR does not require testing because it only adds
infrastructure (command and handler) but doesn’t yet integrate with
devtools actors.
Fixes: Part of #35867 (https://github.com/servo/servo/issues/35867)

---------

Signed-off-by: Uthman Yahaya Baba <uthmanyahayababa@gmail.com>

components/shared/devtools/Cargo.toml

@@ -22,3 +22,4 @@ net_traits = { workspace = true }
 serde = { workspace = true }
 servo_url = { path = "../../url" }
 uuid = { workspace = true, features = ["serde"] }
+embedder_traits = { workspace = true }