Allow trailing commas in bare lists

This applies to macro arguments, DB, DW, DL, DS,
PRINT, PRINTLN, EXPORT, PURGE, and OPT.

It also removes support for empty entries in DB/DW/DL.
(Deprecating it would require keeping parser support,
which is ambiguous with trailing commas.)

Fixes #753

test/asm/macro-arguments.asm

@@ -9,15 +9,20 @@ ENDM
 
 	mac /* block
 		...comment */ ; comment
-	mac /*a*/ 1 , 2 /*b*/
+	mac /*a*/ 1 , 2 /*b*/ , ; trailing comma
 	mac \
 	c, d
 	mac 1, 2 + /* another ;
 		; comment */ 2, 3
 
+	mac           a                                                                                                                                                                                                                                                               b           ; truncated
+
 	mac
+	mac ,
+	mac a,
 	mac a,,
 	mac ,,z
 	mac a,,z
 	mac ,a,b,c,
-	mac ,,x,,
+	mac ,,x,,,
+	mac E,O,F ; no newline