Add .editorconfig

[Iron-E]

Nvim 0.9 is adding support for editorconfigs, which will help in establishing the repo's conventions.

Users <0.9 can use editorconfig.nvim

[romgrk]

I've never seen the interest of editorconfig for neovim. Neovim forces users to learn a lot about it to use it, I don't see the need to support a small simplistic config format in addition to its already existing config options.

[Iron-E]

I wrote one in #422— 0.9 supports it so in worst case if users haven't updated, their editor will safely ignore it. Best case, they won't have to futz with their Neovim config / install something like indent-o-matic in order to contribute with our basic settings.

We can even extend it to show lint messages, e.g.: