patch 9.1.1166: command-line auto-completion hard with wildmenu

Problem:  command-line auto-completion hard with wildmenu
Solution: implement "noselect" wildoption value (Girish Palya)

When `noselect` is present in `wildmode` and 'wildmenu' is enabled, the
completion menu appears without pre-selecting the first item.

This change makes it easier to implement command-line auto-completion,
where the menu dynamically appears as characters are typed, and `<Tab>`
can be used to manually select an item. This can be achieved by
leveraging the `CmdlineChanged` event to insert `wildchar(m)`,
triggering completion menu.

Without this change, auto-completion using the 'wildmenu' mechanism is
not feasible, as it automatically inserts the first match, preventing
dynamic selection.

The following Vimscript snippet demonstrates how to configure
auto-completion using `noselect`:

```vim
vim9script
set wim=noselect:lastused,full wop=pum wcm=<C-@> wmnu
autocmd CmdlineChanged : timer_start(0, function(CmdComplete, [getcmdline()]))

def CmdComplete(cur_cmdline: string, timer: number)
  var [cmdline, curpos] = [getcmdline(), getcmdpos()]
  if cur_cmdline ==# cmdline  # Avoid completing each character in keymaps and pasted text
    && !pumvisible() && curpos == cmdline->len() + 1

    if cmdline[curpos - 2] =~ '[\w*/:]'  # Reduce noise by completing only selected characters
      feedkeys("\<C-@>", "ti")
      set eventignore+=CmdlineChanged  # Suppress redundant completion attempts
      timer_start(0, (_) => {
        getcmdline()->substitute('\%x00$', '', '')->setcmdline()  # Remove <C-@> if no completion items exist
        set eventignore-=CmdlineChanged
      })
    endif
  endif
enddef
```

fixes: #16551
closes: #16759

Signed-off-by: Girish Palya <girishji@gmail.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>

src/testdir/gen_opt_test.vim

@@ -321,6 +321,7 @@ let test_values = {
       \		'bs'],
       \		['xxx']],
       \ 'wildmode': [['', 'full', 'longest', 'list', 'lastused', 'list:full',
+      \		'noselect', 'noselect,full', 'noselect:lastused,full',
       \		'full,longest', 'full,full,full,full'],
       \		['xxx', 'a4', 'full,full,full,full,full']],
       \ 'wildoptions': [['', 'tagfile', 'pum', 'fuzzy'], ['xxx']],