LibJS: Implement null and undefined literals

Author: https://github.com/0xtechnobabble
Commit: cfd710eb31
Pull-request: https://github.com/SerenityOS/serenity/pull/1470
Reviewed-by: https://github.com/awesomekling
Reviewed-by: https://github.com/oriko1010

Libraries/LibJS/Token.cpp

@@ -183,6 +183,8 @@ const char* Token::name(TokenType type)
         return "Try";
     case TokenType::Typeof:
         return "Typeof";
+    case TokenType::UndefinedLiteral:
+        return "UndefinedLiteral";
     case TokenType::UnsignedShiftRight:
         return "UnsignedShiftRight";
     case TokenType::UnsignedShiftRightEquals: