vim-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: vim/vim#16551
closes: vim/vim#16759

2bacc3e5fb

Cherry-pick Wildmode_Tests() change from patch 9.0.0418.

Co-authored-by: Girish Palya <girishji@gmail.com>
Signed-off-by: Tomas Slusny <slusnucky@gmail.com>

src/nvim/cmdexpand.h

@@ -40,6 +40,7 @@ enum {
   WILD_NOERROR              = 0x800,  ///< sets EW_NOERROR
   WILD_BUFLASTUSED          = 0x1000,
   BUF_DIFF_FILTER           = 0x2000,
+  WILD_KEEP_SOLE_ITEM       = 0x4000,
 };
 
 #ifdef INCLUDE_GENERATED_DECLARATIONS