Currency 0.1.1: Carthage Support

Add support for Carthage installation

Motivation:

While SPM is the preferred method for pulling in Swift-only libraries, it's support and capabilities are still sometimes short compared to other dependency managers such as Carthage.

Modifications:

- Add shared Xcode project scheme and file.
- Add github action to verify Carthage builds fine
- Rename `swift.yml` workflow to `tests.yml` to properly break up different github action workflows

Result:

Developers should now be able to use either SPM or Carthage to install swift-currency