LibJS: Allow reserved words as keys in object expressions.

Author: https://github.com/sunverwerth
Commit: bf5b251684
Pull-request: https://github.com/SerenityOS/serenity/pull/1849
Issue: https://github.com/SerenityOS/serenity/issues/1768

Libraries/LibJS/Token.h

@@ -153,6 +153,8 @@ public:
     String string_value() const;
     bool bool_value() const;
 
+    bool is_identifier_name() const;
+
 private:
     TokenType m_type;
     StringView m_trivia;