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)
diff --git a/src/version.c b/src/version.c
index 7cc9e7b..b6cb477 100644
--- a/src/version.c
+++ b/src/version.c
@@ -751,6 +751,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    1723,
+/**/
     1722,
 /**/
     1721,