Introduce the initial version of the swift

[hermet]

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

Need to confirm:

• [ ] Replace the thorvg core with the git submodule
• [ ] Align the APIs, possibly with other projects (Android, Flutter, Web)

[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. 👍