Make setup scripts cross-platform

[nathanshelly]

anyone on macOS or Linux (or Windows Subsystem for Linux) should be able to run make setup without issues

will involve changes from the docker branch

commits:

• https://github.com/nathanshelly/.files/commit/162a885cf9aa41efe49e1f75de0fd6285c22bd8a
• https://github.com/nathanshelly/.files/commit/60bbb854fd05a5c1bb420df9f60b41a1e2503ace

[nathanshelly]

some things to deal with:

• [x] Homebrew installation - done by https://github.com/nathanshelly/.files/commit/0b50ea2815bd5f432329fe4b86abf9481519e07a
• [ ] Homebrew loading/adding to path
• [x] Move macOS stuff into macOS folder - done in https://github.com/nathanshelly/.files/pull/66