Support SPM as a build tool

[pepicrft]

First of all, thanks for building and maintaining this project.

At Tuist we are starting to explore what value we could provide to developers by integrating into that contract and for that we are pondering to use this package.

Since we don't use Bazel as a build system, I wonder if you'd be up for adding a Package.swift, which we can use to resolve this dependency downstream using SPM.