Define a Podspec

[liamnichols]

Background

We'll also want to deploy using CocoaPods

Description

• Define SwiftUserDefaults.podspec to allow usage with CocoaPods.
• Add a workflow for publishing if we are ready to make the repo public.
• Add tests/validation for pods (library and example project)

[liamnichols]

I defined a similar pod spec for a different project here: https://github.com/liamnichols/ListItemFormatter/blob/main/ListItemFormatter.podspec

In addition, it is worth having CI validate the spec like this: https://github.com/liamnichols/ListItemFormatter/blob/942a51c5eeec3f5834618230df4df290c8eecac8/.github/workflows/ci.yml#L57-L59

[liamnichols]

Thanks @ryanpato! 🚀