Implement macOS support

[patrickfreed]

This is a tracking issue for macOS support. More details will be added as that work is scoped out.

Once Linux has reached MVP status, this will be addressed.

[iainsmith]

HI @patrickfreed, I was looking at this quickly and was wondering what's needed for this:

• Update Package.swift
• Implement MacPlatform.swift and struct Mac: Platform (or would you prefer Darwin) as the name here?

I'd be keen to look at an existing branch, if one exists.

[sdavids]

swiftly/DESIGN.md

Line 57 in 364b02b

Similar to Linux, the bootstrapping script for macOS will create a `~/.swiftly/bin` directory and drop the swiftly executable in it. A similar `~/.swiftly/env` file will be created and a message will be printed suggesting users add source `~/.swiftly/env` to their `.bash_profile` or `.zshrc`.

Please honor XDG conventions on MacOS also—Mac users prefer uncluttered home directories too 😉

Here is how swiftpm did it: swiftlang/swift-package-manager#7386

[cmcgee1024]

This has been implemented in #121