Add proof-of-concept completion with info (Vim 8.2+)

[jsit]

Proof of concept for adding completion "info" for CSS properties/values.

:h complete-items

Demo:

1. Open vim --clean
2. Load csscomplete.vim (e.g. :so ~/.vim/bundle/csscomplete.vim/autoload/csscomplete.vim)
3. Add "popup" to completion options (:set completeopt+=popup)
4. Turn on filetype plugins (:filetype plugin on)
5. Set filetype to CSS (:set ft=css)
6. In insert mode, type al<C-X><C-O>

Omni Completion menu appears, with additional popup window showing definition of all CSS property:

"Info" text could be sourced from VSCode