Problem: no support for custom cmdline completion
Solution: Add new vimscript functions
Add the following two functions:
- getcmdcompltype() returns custom and customlist functions
- getcompletion() supports both custom and customlist
closes: #12228
Signed-off-by: Christian Brabandt <cb@256bit.org>
Co-authored-by: Shougo Matsushita <Shougo.Matsu@gmail.com>