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/eval.lua

@@ -13069,6 +13069,36 @@ M.funcs = {
     params = {},
     signature = 'wildmenumode()',
   },
+  wildtrigger = {
+    desc = [==[
+      Start wildcard expansion in the command-line, using the
+      behavior defined by the 'wildmode' and 'wildoptions' settings.
+      See |cmdline-completion|.
+
+      This function also enables completion in search patterns such
+      as |/|, |?|, |:s|, |:g|, |:v| and |:vimgrep|.
+
+      Unlike pressing 'wildchar' manually, this function does not
+      produce a beep when no matches are found and generally
+      operates more quietly.  This makes it suitable for triggering
+      completion automatically, such as from an |:autocmd|.
+      					*cmdline-autocompletion*
+      Example: To make the completion menu pop up automatically as
+      you type on the command line, use: >vim
+      	autocmd CmdlineChanged [:/?] call wildtrigger()
+      	set wildmode=noselect:lastused,full wildoptions=pum
+      <
+      To retain normal history navigation (up/down keys): >vim
+      	cnoremap <Up>   <C-U><Up>
+      	cnoremap <Down> <C-U><Down>
+      <
+      Return value is always 0.
+    ]==],
+    name = 'wildtrigger',
+    params = {},
+    returns = 'number',
+    signature = 'wildtrigger()',
+  },
   win_execute = {
     args = { 2, 3 },
     base = 2,