runtime(vim): accept dot in completionList for Vim9 syntax (vim/vim#13832)
if you have autoloaded command complete function in vim9script, it shouldn't be highlighted as Error:
```
import autoload "share.vim"
command! -range=% -nargs=? -complete=custom,share.Complete Share share.Paste(<q-args>, <line1>, <line2>)
```
`share.Complete` is a valid complete function.
ceed36873e
Co-authored-by: Maxim Kim <habamax@gmail.com>
5de14b7206
