vim-patch:9.1.0568: Cannot expand paths from 'cdpath' setting

Problem:  Cannot expand paths from 'cdpath' setting
          (Daniel Hahler)
Solution: Implement 'cdpath' completion, add the new 'dir_in_path'
          completion type (LemonBoy)

fixes vim/vim#374
closes: vim/vim#15205

a20bf69a3b

Co-authored-by: LemonBoy <thatlemon@gmail.com>

src/nvim/usercmd.c

@@ -98,6 +98,7 @@ static const char *command_complete[] = {
   [EXPAND_USER_VARS] = "var",
   [EXPAND_BREAKPOINT] = "breakpoint",
   [EXPAND_SCRIPTNAMES] = "scriptnames",
+  [EXPAND_DIRS_IN_CDPATH] = "dir_in_path",
 };
 
 /// List of names of address types.  Must be alphabetical for completion.