patch 8.2.1718: Vim9: :def function disallows "firstline" for no good reason

Problem:    Vim9: :def function disallows "firstline" and "lastline" argument
            names for no good reason.
Solution:   Don't check the arguments for a :def function. (closes #6986)

src/version.c

@@ -750,6 +750,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    1718,
 /**/
     1717,
 /**/