patch 8.2.2449: Vim9: flatten() always changes the list type

Problem:    Vim9: flatten() always changes the list type.
Solution:   Disallow using flatten() and add flattennew().
diff --git a/src/evalfunc.c b/src/evalfunc.c
index f0a15f9..0b4b502 100644
--- a/src/evalfunc.c
+++ b/src/evalfunc.c
@@ -954,6 +954,8 @@
 			ret_string,	    f_findfile},
     {"flatten",		1, 2, FEARG_1,	    NULL,
 			ret_list_any,	    f_flatten},
+    {"flattennew",	1, 2, FEARG_1,	    NULL,
+			ret_list_any,	    f_flattennew},
     {"float2nr",	1, 1, FEARG_1,	    NULL,
 			ret_number,	    FLOAT_FUNC(f_float2nr)},
     {"floor",		1, 1, FEARG_1,	    NULL,