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/cmdexpand_defs.h

@@ -105,6 +105,7 @@ enum {
   EXPAND_SETTING_SUBTRACT,
   EXPAND_ARGOPT,
   EXPAND_KEYMAP,
+  EXPAND_DIRS_IN_CDPATH,
   EXPAND_CHECKHEALTH,
   EXPAND_LUA,
 };