create u+ list for parsing escape characters

Author: xiaohe27

JavaFrontParser/syntax/languages/java-15/lexical/UnicodeEscapes.sdf

@@ -9,4 +9,5 @@ exports
     UnicodeEscape
 
   syntax
-    "\\" [u]+ [0-9a-fA-F] [0-9a-fA-F] [0-9a-fA-F] [0-9a-fA-F] -> UnicodeEscape {cons("UnicodeEscape")}
+    "\\" UList [0-9a-fA-F] [0-9a-fA-F] [0-9a-fA-F] [0-9a-fA-F] -> UnicodeEscape {cons("UnicodeEscape")}
+    [u]+ -> UList {cons("UList")}