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/testdir/test_vim9_func.vim

@@ -372,6 +372,15 @@ def Test_call_def_varargs()
   END
   CheckScriptSuccess(lines)
 
+  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>)

src/userfunc.c

@@ -211,7 +211,7 @@ get_function_args(
 	    if (argtypes != NULL)
 	    {
 		// ...name: list<type>
-		if (!ASCII_ISALPHA(*p))
+		if (!eval_isnamec1(*p))
 		{
 		    emsg(_(e_missing_name_after_dots));
 		    break;

src/version.c

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