LibJS: Disallow 'return' outside of a function

Author: https://github.com/mattco98
Commit: 9a82c22a85
Pull-request: https://github.com/SerenityOS/serenity/pull/3711
Issue: https://github.com/SerenityOS/serenity/issues/3607

Libraries/LibJS/Parser.h

@@ -158,6 +158,7 @@ private:
         bool m_strict_mode { false };
         bool m_allow_super_property_lookup { false };
         bool m_allow_super_constructor_call { false };
+        bool m_in_function_context { false };
 
         explicit ParserState(Lexer);
     };