devtools: Handle getEnvironment message (#43167)

Initial work to support scopes in the debugger. They don't show at the
moment since we are not processing variables yet. Depends on #43166.

Testing: Ran `mach test-devtools` and manual testing.
Part of: #36027

Signed-off-by: eri <eri@igalia.com>
Co-authored-by: atbrakhi <atbrakhi@igalia.com>

components/script_bindings/webidls/DebuggerGetEnvironmentEvent.webidl

@@ -4,11 +4,19 @@
 
 // This interface is entirely internal to Servo, and should not be accessible to
 // web pages.
+[Exposed=DebuggerGlobalScope]
+interface DebuggerGetEnvironmentEvent : Event {
+    readonly attribute DOMString frameActorId;
+};
+
 partial interface DebuggerGlobalScope {
     DOMString? registerEnvironmentActor(
         EnvironmentInfo result,
         DOMString? parent
     );
+    undefined getEnvironmentResult(
+        DOMString environmentActorId
+    );
 };
 
 dictionary EnvironmentInfo {