Convert project to SPM

[djbe]

Similar to SwiftGen/SwiftGen#793, but much simpler in scope as we already have full SPM support in this project.

This just:

• Removes the old Xcode projects
• Removes CocoaPods (as a dependency/tool)
• Use commit hooks (Komondor) to run SwiftLint
• Adapts Rake and CI as needed

[djbe]

Allright so this is slightly weird, and I'm not sure how we should deal with it:

• With Switch to Github Actions #129 merged, we now run certain jobs only if specific files have changed
• I updated the list of required checks before a merge to those from Switch to Github Actions #129
• But this PR does not change the podspec file, so doesn't trigger a Pod Lint check...

Do we remove that pod check from the list of required steps? What about PRs where there's no code change, such as a doc PR? Should we remove all checks from being required?