patch 8.2.0875: getting attributes for directory entries is slow

Problem:    Getting attributes for directory entries is slow.
Solution:   Add readdirex(). (Ken Takata, closes #5619)
diff --git a/src/evalfunc.c b/src/evalfunc.c
index 92cd4a4..45937fa 100644
--- a/src/evalfunc.c
+++ b/src/evalfunc.c
@@ -767,6 +767,7 @@
     {"rand",		0, 1, FEARG_1,	  ret_number,	f_rand},
     {"range",		1, 3, FEARG_1,	  ret_list_number, f_range},
     {"readdir",		1, 2, FEARG_1,	  ret_list_string, f_readdir},
+    {"readdirex",	1, 2, FEARG_1,	  ret_list_dict_any, f_readdirex},
     {"readfile",	1, 3, FEARG_1,	  ret_any,	f_readfile},
     {"reg_executing",	0, 0, 0,	  ret_string,	f_reg_executing},
     {"reg_recording",	0, 0, 0,	  ret_string,	f_reg_recording},