LibJS: Add ECMAScriptFunctionObject forward declaration

Author: https://github.com/linusg
Commit: ce946dd656
Pull-request: https://github.com/SerenityOS/serenity/pull/10203
Reviewed-by: https://github.com/IdanHo ✅
Reviewed-by: https://github.com/davidot

Userland/Libraries/LibJS/Forward.h

@@ -145,6 +145,7 @@ class Completion;
 class Console;
 class DeclarativeEnvironment;
 class DeferGC;
+class ECMAScriptFunctionObject;
 class Environment;
 class Error;
 class ErrorType;