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/version9.txt

@@ -1,4 +1,4 @@
-*version9.txt*  For Vim version 9.1.  Last change: 2025 Jul 20
+*version9.txt*  For Vim version 9.1.  Last change: 2025 Jul 21
 
 
 		  VIM REFERENCE MANUAL    by Bram Moolenaar
@@ -41601,6 +41601,7 @@ Completion~
 	"nosort"	- do not sort completion results
 	"preinsert"	- highlight to be inserted values
 	"nearest"	- sort completion results by distance to cursor
+- new function |wildtrigger()| to trigger wildcard expansion
 
 Platform specific~
 -----------------
@@ -41765,6 +41766,7 @@ Functions: ~
 |str2blob()|		convert a List of strings into a blob
 |test_null_tuple()|	return a null tuple
 |tuple2list()|		turn a Tuple of items into a List
+|wildtrigger()|		trigger wildcard expansion
 
 
 Autocommands: ~