Add nvim-cmp kind highlights

[mcritchlow]

This is a relatively new feature that was added to nvim-cmp

see: https://github.com/hrsh7th/nvim-cmp/wiki/Menu-Appearance#how-to-add-visual-studio-code-dark-theme-colors-to-the-menu

I'd love to have this built-in to this theme (which I use daily, thanks!)

Not sure if it's helpful, but here's a referenced PR to add similar theme
functionality to a different theme:

tomasiser/vim-code-dark#87

Thanks for considering.

[mcritchlow]

@ful1e5 - just wanted to say thanks for adding this, especially so quickly! Much appreciated, it looks great :)