Add Protolint

[fredrikaverpil]

Hi @mfussenegger and thanks for this the awesome nvim-lint project! 👋

Here's a simple but functioning implementation for protolint. 😄

This is how I've implemented it locally, and which shows warnings just fine: https://github.com/fredrikaverpil/dotfiles/commit/44fe5b5e560077764a2b2f78b9ca33591f489e24

[mfussenegger]

Thanks