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/options.txt

@@ -1,4 +1,4 @@
-*options.txt*	For Vim version 9.1.  Last change: 2025 Jul 20
+*options.txt*	For Vim version 9.1.  Last change: 2025 Jul 21
 
 
 		  VIM REFERENCE MANUAL	  by Bram Moolenaar
@@ -9797,7 +9797,7 @@ A jump table for the options with a short description can be found at |Q_op|.
 <	'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()|.
 	NOTE: This option is set to the Vi default value when 'compatible' is
 	set and to the Vim default value when 'compatible' is reset.