vim-patch:9.1.0770: current command line completion is a bit limited (#30728)

Problem:  current command completion is a bit limited
Solution: Add the shellcmdline completion type and getmdcomplpat()
          function (Ruslan Russkikh).

closes: vim/vim#15823

0407d621bb

Co-authored-by: Ruslan Russkikh <dvrussk@yandex.ru>

src/nvim/cmdexpand_defs.h

@@ -106,6 +106,7 @@ enum {
   EXPAND_ARGOPT,
   EXPAND_KEYMAP,
   EXPAND_DIRS_IN_CDPATH,
+  EXPAND_SHELLCMDLINE,
   EXPAND_CHECKHEALTH,
   EXPAND_LUA,
 };