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/usr_41.txt

@@ -1,4 +1,4 @@
-*usr_41.txt*	For Vim version 9.1.  Last change: 2025 Jul 05
+*usr_41.txt*	For Vim version 9.1.  Last change: 2025 Jul 21
 
 		     VIM USER MANUAL - by Bram Moolenaar
 
@@ -1233,6 +1233,7 @@ Mappings and Menus:			    *mapping-functions*
 	mapset()		restore a mapping
 	menu_info()		get information about a menu item
 	wildmenumode()		check if the wildmode is active
+	wildtrigger()		start wildcard expansion
 
 Testing:				    *test-functions*
 	assert_equal()		assert that two expressions values are equal