Refactor freezing of JS objects

- Shallow-freezing of objects is never used in our codebase, so remove
   that code path.
 - Deep-freeze is bugged in recent versions of SpiderMonkey (see bug
   https://bugzilla.mozilla.org/show_bug.cgi?id=1930258). Until a fix
   and/or a better API is provided, reimplement this feature by
   recusively freezing object properties.
 - Add tests for the deepfreeze function.

source/scriptinterface/ScriptInterface.cpp

@@ -214,7 +214,7 @@ JS::Value deepfreeze(const ScriptInterface& scriptInterface, JS::HandleValue val
 		return JS::UndefinedValue();
 	}
 
-	Script::FreezeObject(rq, val, true);
+	Script::DeepFreezeObject(rq, val);
 	return val;
 }