Introduce the initial version of the Swift package

[hermet]

Migration from: https://github.com/andyf-canva/thorvg-swift

TODO:

• Replace the thorvg core with the git submodule (Migrate Swift code from thorvg-swift #2 )
• Migrate core code from thorvg-swift (Migrate Swift code from thorvg-swift #2)
• Align the APIs, possibly with other projects (Android, Flutter, Web) (Migrate Swift code from thorvg-swift #2)
• Create SwiftUI and UIKit views that wrap LottieRenderer to promote wider developer adoption #3

[andyf-canva]

@hermet from the thorvg-swift repo, I propose the following files be migrated:

• Package.swift
• Package.resolved
• swift/
  • Animation.swift
  • Canvas.swift
  • Engine.swift
  • Lottie.swift
  • LottieRenderer.swift
  • Picture.swift
  • PixelFormat.swift
  • ThorVGError.swift
• swift-tests/
  • LottieRendererTests.swift
  • LottieTests.swift
  • Resources/test.json
  • SnapshotTests/
    • __Snapshots__/LottieSnapshotTests/*
    • LottieSnapshotTests.swift

Are you happy for me to open a PR containing these files?
I'll also get the thorvg submodule working so we can check that the Swift Package builds.

From there, the next step could be to address aligning the APIs as you've mentioned in your comment above, to firm up an initial public release. Keen to hear your thoughts.

[andyf-canva]

A heads up: I've opened up two PRs as you can see above. Happy to discuss when ready to. 👍