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