patch 8.2.2750: Vim9: error for using underscore in nested function

Problem:    Vim9: error for using underscore in nested function.
Solution:   Do not consider "_" already defined. (closes #8096)

src/testdir/test_vim9_func.vim

@@ -2637,6 +2637,8 @@ def Test_ignored_argument()
         return _
       endfunc
       assert_equal('too', Oktoo())
+
+      assert_equal([[1], [2], [3]], range(3)->mapnew((_, v) => [v]->map((_, w) => w + 1)))
   END
   CheckScriptSuccess(lines)
 

src/version.c

@@ -750,6 +750,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    2750,
 /**/
     2749,
 /**/

src/vim9compile.c

@@ -422,6 +422,10 @@ check_defined(char_u *p, size_t len, cctx_T *cctx, int is_arg)
     int		c = p[len];
     ufunc_T	*ufunc = NULL;
 
+    // underscore argument is OK
+    if (len == 1 && *p == '_')
+	return OK;
+
     if (script_var_exists(p, len, cctx) == OK)
     {
 	if (is_arg)