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>

src/vim.h

@@ -897,6 +897,7 @@ extern int (*dyn_libintl_wputenv)(const wchar_t *envstring);
 #define BUF_DIFF_FILTER		    0x2000
 #define WILD_KEEP_SOLE_ITEM	    0x4000
 #define WILD_MAY_EXPAND_PATTERN	    0x8000
+#define WILD_FUNC_TRIGGER	    0x10000 // called from wildtrigger()
 
 // Flags for expand_wildcards()
 #define EW_DIR		0x01	// include directory names