Fixed trailing underscores in numeric literals (#1264)

This change prohibits such illegal numeric literals as 13_37_ or 0xCAFE_BABE_ (note the trailing underscore).

grammar/src/lexical.grm

@@ -7,7 +7,9 @@ Digit
   : ["0".."9"];
 
 IntegerLiteral
-  : Digit (Digit | "_")*
+  : Digit
+  : Digit (Digit | "_")* Digit
+  ;
 
 FloatLiteral
   : <Java double literal>;
@@ -17,7 +19,9 @@ HexDigit
   : Digit | ["A".."F", "a".."f"];
 
 HexadecimalLiteral
-  : "0x" HexDigit (HexDigit | "_")*;
+  : "0x" HexDigit
+  : "0x" HexDigit (HexDigit | "_")* HexDigit
+  ;
 
 CharacterLiteral
   : <character as in Java>;
@@ -136,4 +140,4 @@ internal
 protected
 catch
 finally
-*/
+*/