runtime(vim): accept dot in completionList for Vim9 syntax (#13832)

runtime(vim): accept dot in completionList for Vim9 syntax (#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.

Signed-off-by: Maxim Kim <habamax@gmail.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>