Target more platforms with Theos

[kabiroberai]

While regular SPM projects that use Orion should work on any Apple platform, Theos in particular only supports iOS at the moment. It would be useful to support other platforms like macOS and the iOS simulator.

This will probably require building multiple copies of the framework and selecting the right one depending on the platform. Something similar will have to be figured out for the tweak_swift template's Package.swift as well – maybe a binaryTarget, or some modulemap trickery.