Add support for Swift Package Manager

[sindresorhus]

Instead of having to bundle the Xcode project. Swift Package Manager supports libraries: https://rderik.com/blog/making-a-c-library-available-in-swift-using-the-swift-package/

To be clear. I don't mean adding Swift code here. Just a Package.swift and a module map.

This would make it so much nicer to import gifski into Gifski.app and make it easier to update.

[kornelski]

It'd be very nice to have SPM compatibility.

I haven't had time to learn it yet, and it's unlikely I'll get around to it anytime soon, but PRs are welcome.

[XMLHexagram]

Hello, I have created a new repo called GifskiFramework which provides support for the Swift package manager. The new repo uses many build scripts and a complex GitHub action to automatically build a SwiftPM.

@kornelski Please let me know if you would be happy to have a PR that adds these new files to this repo or any other acceptable ways to add SwiftPM support. I am always glad to help.