Set up editor config

[doug-wade]

I really like the idea of editorconfig, and I use the GitHub text editor to draft quick prs on the go. I think we should add a .editorconfig with some rules from the repo. Here's their documentation https://editorconfig.org/ if you want to find out more.

I think we can start with the config:

[*]
indent_style: space
indent_size: 4
charset: utf-8
trim_trailing_whitespace: true
insert_final_newline: true
max_line_length: 120