vim-patch:9.1.1576: cannot easily trigger wildcard expansion (#35022)

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 vim/vim#16759.

closes: vim/vim#17806

b486ed8266

While at it, also make Ctrl-Z trigger search completion.

Co-authored-by: Girish Palya <girishji@gmail.com>

src/nvim/keycodes.h

@@ -215,10 +215,15 @@ enum key_extra {
   // KE_FOCUSGAINED = 98,   // focus gained
   // KE_FOCUSLOST = 99,     // focus lost
   KE_MOUSEMOVE = 100,       // mouse moved with no button down
-  // KE_CANCEL = 101,       // return from vgetc()
+  // KE_MOUSEMOVE_XY = 101,
+  // KE_CANCEL = 102,       // return from vgetc()
   KE_EVENT = 102,           // event
   KE_LUA = 103,             // Lua special key
   KE_COMMAND = 104,         // <Cmd> special key
+  // KE_S_BS = 105,
+  // KE_SID = 106,
+  // KE_ESC = 107,
+  KE_WILD = 108,            // triggers wildmode completion
 };
 
 // the three byte codes are replaced with the following int when using vgetc()
@@ -451,6 +456,8 @@ enum key_extra {
 #define K_COMMAND       TERMCAP2KEY(KS_EXTRA, KE_COMMAND)
 #define K_LUA           TERMCAP2KEY(KS_EXTRA, KE_LUA)
 
+#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