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/keymap.h

@@ -279,6 +279,7 @@ enum key_extra
     , KE_S_BS = 105		// shift + <BS>
     , KE_SID = 106		// <SID> special key, followed by {nr};
     , KE_ESC = 107		// used for K_ESC
+    , KE_WILD = 108		// triggers wildmode completion
 };
 
 /*
@@ -491,6 +492,8 @@ enum key_extra
 #define K_SCRIPT_COMMAND TERMCAP2KEY(KS_EXTRA, KE_SCRIPT_COMMAND)
 #define K_SID		TERMCAP2KEY(KS_EXTRA, KE_SID)
 
+#define K_WILD		TERMCAP2KEY(KS_EXTRA, KE_WILD)
+
 // Bits for modifier mask
 // 0x01 cannot be used, because the modifier must be 0x02 or higher
 #define MOD_MASK_SHIFT	    0x02