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/testdir/test_vim9_func.vim b/src/testdir/test_vim9_func.vim
index 4051650..e6f40cc 100644
--- a/src/testdir/test_vim9_func.vim
+++ b/src/testdir/test_vim9_func.vim
@@ -374,6 +374,15 @@
 
   lines =<< trim END
       vim9script
+      def Func(..._l: list<string>)
+        echo _l
+      enddef
+      Func('a', 'b', 'c')
+  END
+  CheckScriptSuccess(lines)
+
+  lines =<< trim END
+      vim9script
       def Func(...l: list<string>)
         echo l
       enddef
diff --git a/src/userfunc.c b/src/userfunc.c
index be2a18c..eb9dadf 100644
--- a/src/userfunc.c
+++ b/src/userfunc.c
@@ -211,7 +211,7 @@
 	    if (argtypes != NULL)
 	    {
 		// ...name: list<type>
-		if (!ASCII_ISALPHA(*p))
+		if (!eval_isnamec1(*p))
 		{
 		    emsg(_(e_missing_name_after_dots));
 		    break;
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,