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>

runtime/lua/vim/_meta/vimfn.lua

@@ -2871,7 +2871,9 @@ function vim.fn.getcmdwintype() end
 --- 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