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/options.lua

@@ -10173,7 +10173,7 @@ local options = {
         <	'wildchar' also enables completion in search pattern contexts such as
         |/|, |?|, |:s|, |:g|, |:v|, and |:vim|.  To insert a literal <Tab>
         instead of triggering completion, type <C-V><Tab> or "\t".
-        See also |'wildoptions'|.
+        See also 'wildoptions' and |wildtrigger()|.
       ]=],
       full_name = 'wildchar',
       scope = { 'global' },