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>

runtime/lua/vim/_meta/options.lua

@@ -7865,7 +7865,7 @@ vim.go.ww = vim.go.whichwrap
 --- '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()`.
 ---
 --- @type integer
 vim.o.wildchar = 9