patch 9.1.1576: cannot easily trigger wildcard expansion

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 #16759.

closes: #17806

Signed-off-by: Girish Palya <girishji@gmail.com>
Co-authored-by: zeertzjq <zeertzjq@outlook.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>

runtime/doc/builtin.txt

@@ -1,4 +1,4 @@
-*builtin.txt*	For Vim version 9.1.  Last change: 2025 Jul 17
+*builtin.txt*	For Vim version 9.1.  Last change: 2025 Jul 21
 
 
 		  VIM REFERENCE MANUAL	  by Bram Moolenaar
@@ -757,6 +757,7 @@ virtcol2col({winid}, {lnum}, {col})
 				Number  byte index of a character on screen
 visualmode([{expr}])		String	last visual mode used
 wildmenumode()			Number	whether 'wildmenu' mode is active
+wildtrigger()			Number	start wildcard expansion
 win_execute({id}, {command} [, {silent}])
 				String	execute {command} in window {id}
 win_findbuf({bufnr})		List	find windows containing {bufnr}
@@ -12331,6 +12332,33 @@ wildmenumode()					*wildmenumode()*
 		Return type: |Number|
 
 
+wildtrigger()						*wildtrigger()*
+		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: >
+			autocmd CmdlineChanged [:/?] call wildtrigger()
+			set wildmode=noselect:lastused,full wildoptions=pum
+<
+		To retain normal history navigation (up/down keys): >
+			cnoremap <Up>   <C-U><Up>
+			cnoremap <Down> <C-U><Down>
+<
+		Return value is always 0.
+
+		Return type: |Number|
+
+
 win_execute({id}, {command} [, {silent}])		*win_execute()*
 		Like `execute()` but in the context of window {id}.
 		The window will temporarily be made the current window,