vim-patch:9.0.1774: no support for custom cmdline completion (#24808)

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: vim/vim#12228

92997dda78

Co-authored-by: Shougo Matsushita <Shougo.Matsu@gmail.com>

src/nvim/eval.lua

@@ -3582,7 +3582,9 @@ M.funcs = {
       color		color schemes
       command		Ex command
       compiler	compilers
-      diff_buffer     |:diffget| and |:diffput| completion
+      custom,{func}	custom completion, defined via {func}
+      customlist,{func} custom completion, defined via {func}
+      diff_buffer	|:diffget| and |:diffput| completion
       dir		directory names
       environment	environment variable names
       event		autocommand events