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/cmdline.txt

@@ -1,4 +1,4 @@
-*cmdline.txt*   For Vim version 9.1.  Last change: 2025 Jun 28
+*cmdline.txt*   For Vim version 9.1.  Last change: 2025 Jul 21
 
 
 		  VIM REFERENCE MANUAL    by Bram Moolenaar
@@ -479,6 +479,8 @@ When repeating 'wildchar' or CTRL-N you cycle through the matches, eventually
 ending up back to what was typed.  If the first match is not what you wanted,
 you can use <S-Tab> or CTRL-P to go straight back to what you typed.
 
+See also |wildtrigger()|.
+
 The 'wildmenu' option can be set to show the matches just above the command
 line.