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/script/script_thread.rs

@@ -2063,6 +2063,14 @@ impl ScriptThread {
             DevtoolScriptControlMsg::GetCssDatabase(reply) => {
                 devtools::handle_get_css_database(reply)
             },
+            DevtoolScriptControlMsg::SimulateColorScheme(id, theme) => {
+                match documents.find_window(id) {
+                    Some(window) => {
+                        window.handle_theme_change(theme);
+                    },
+                    None => warn!("Message sent to closed pipeline {}.", id),
+                }
+            },
         }
     }