patch 9.0.1774: no support for custom cmdline completion

Problem:  no support for custom cmdline completion
Solution: Add new vimscript functions

Add the following two functions:
- getcmdcompltype() returns custom and customlist functions

- getcompletion() supports both custom and customlist

closes: #12228

Signed-off-by: Christian Brabandt <cb@256bit.org>
Co-authored-by: Shougo Matsushita <Shougo.Matsu@gmail.com>
diff --git a/src/usercmd.c b/src/usercmd.c
index 57435fa..3191257 100644
--- a/src/usercmd.c
+++ b/src/usercmd.c
@@ -481,6 +481,11 @@
 {
     int i;
 
+    if (STRNCMP(complete_str, "custom,", 7) == 0)
+	return EXPAND_USER_DEFINED;
+    if (STRNCMP(complete_str, "customlist,", 11) == 0)
+	return EXPAND_USER_LIST;
+
     for (i = 0; command_complete[i].expand != 0; ++i)
 	if (STRCMP(complete_str, command_complete[i].name) == 0)
 	    return command_complete[i].expand;