chromatica.nvim support

[mklein994]

I've recently discovered chromatica.nvim, a semantic highlighting plugin for Neovim for C-family languages. It would be great if gruvbox supported this plugin.

About the screenshot: left and right is before and after :ChromaticaStart.

The plugin uses clang as a backend, and asynchronously provides syntax groups to extend highlighting. As you may notice, variables (linked to the Variable highlight group) are colored Grey, which is IMHO too close to the Comment highlight group. There's might be other elements you want to tweak, but I'll leave that up to you.

I don't know much about creating color schemes for vim, but if you have any other questions let me know.