vim-patch:9.1.0568: Cannot expand paths from 'cdpath' setting

Problem:  Cannot expand paths from 'cdpath' setting
          (Daniel Hahler)
Solution: Implement 'cdpath' completion, add the new 'dir_in_path'
          completion type (LemonBoy)

fixes vim/vim#374
closes: vim/vim#15205

a20bf69a3b

Co-authored-by: LemonBoy <thatlemon@gmail.com>

runtime/lua/vim/_meta/vimfn.lua

@@ -2942,6 +2942,7 @@ function vim.fn.getcmdwintype() end
 --- customlist,{func} custom completion, defined via {func}
 --- diff_buffer  |:diffget| and |:diffput| completion
 --- dir    directory names
+--- dir_in_path  directory names in |'cdpath'|
 --- environment  environment variable names
 --- event    autocommand events
 --- expression  Vim expression