vim-patch:9.1.1576: cannot easily trigger wildcard expansion (#35022)

Problem:  cannot easily trigger wildcard expansion
Solution: Introduce wildtrigger() function
          (Girish Palya)

This PR introduces a new `wildtrigger()` function.

See `:h wildtrigger()`

`wildtrigger()` behaves like pressing the `wildchar,` but provides a
more refined and controlled completion experience:

- Suppresses beeps when no matches are found.
- Avoids displaying irrelevant completions (like full command lists)
  when the prefix is insufficient or doesn't match.
- Skips completion if the typeahead buffer has pending input or if a
  wildmenu is already active.
- Does not print "..." before completion.

This is an improvement on the `feedkeys()` based autocompletion script
given in vim/vim#16759.

closes: vim/vim#17806

b486ed8266

While at it, also make Ctrl-Z trigger search completion.

Co-authored-by: Girish Palya <girishji@gmail.com>

src/nvim/cmdexpand.h

@@ -42,6 +42,7 @@ enum {
   BUF_DIFF_FILTER           = 0x2000,
   WILD_KEEP_SOLE_ITEM       = 0x4000,
   WILD_MAY_EXPAND_PATTERN   = 0x8000,
+  WILD_FUNC_TRIGGER         = 0x10000,  ///< called from wildtrigger()
 };
 
 #ifdef INCLUDE_GENERATED_DECLARATIONS