[misc] Introduce .editorconfig

Summary:
Set default indent style to space with default indent size of 2

(Additional rules can be put up as separate diffs)

Worked with VSCode after installing EditorConfig extension: https://marketplace.visualstudio.com/items?itemName=EditorConfig.EditorConfig

It looks like the nano/vim/emacs people will need to install some plugins to get it to work for them

Test Plan:
Here it is working in VSCode (did a python file so existing eslint/prettier/whatever config wouldn't interfere)

Reviewers: jimpo, ashoat, palys-swm, def-au1t, varun, benschac

Reviewed By: ashoat

Subscribers: Adrian, karol-bisztyga

Differential Revision: https://phabricator.ashoat.com/D3427