patch 8.2.1723: Vim9: Variable argument name cannot start with underscore

Problem:    Vim9: Variable argument name cannot start with underscore.
Solution:   Use eval_isnamec1(). (closes #6988)

src/version.c

@@ -750,6 +750,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    1723,
 /**/
     1722,
 /**/