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>

runtime/syntax/vim.vim

@@ -99,7 +99,7 @@ syn match	vimUserAutoEvent	contained	"\<\h\w*\>"	skipwhite nextgroup=vimUserAuto
 
 " Highlight commonly used Groupnames {{{2
 " GEN_SYN_VIM: vimGroup, START_STR='syn keyword vimGroup contained', END_STR=''
-syn keyword vimGroup contained Added Boolean Changed Character Comment Conditional Constant Debug Define Delimiter Error Exception Float Function Identifier Ignore Include Keyword Label Macro Number Operator PreCondit PreProc Removed Repeat Special SpecialChar SpecialComment Statement StorageClass String Structure Tag Todo Type Typedef Underlined
+syn keyword vimGroup contained Added Bold BoldItalic Boolean Changed Character Comment Conditional Constant Debug Define Delimiter Error Exception Float Function Identifier Ignore Include Italic Keyword Label Macro Number Operator PreCondit PreProc Removed Repeat Special SpecialChar SpecialComment Statement StorageClass String Structure Tag Todo Type Typedef Underlined
 
 " Default highlighting groups {{{2
 " GEN_SYN_VIM: vimHLGroup, START_STR='syn keyword vimHLGroup contained', END_STR=''