Rework configuration

[gegoune]

Just a placeholder for now, will close if won't gain friction.

We could look into overloaded (multi type) settings, like show_root_label and break it into structs such as
show_root = { enabled = true, label = ...}.

[alex-courtis]

This will deal with a few issues / questions that have recently come up.

I've also been thinking about a config class, similar to our API parameter classes. Inspiration: https://github.com/neovim/nvim-lspconfig/blob/95b2fc427353e42318c974d10685d500441b821b/lua/lspconfig/configs.lua