watchOS support

[noahsmartin]

I noticed the Package.swift does not support watchOS, but the code seems to build without a problem on watchOS, this commit adds it and the result worked fine at least for how I was using it: EmergeTools@30a96b4

I didn't test all features, just the ones I'm using for SnapshotPreviews here: https://github.com/EmergeTools/SnapshotPreviews-iOS/blob/main/Sources/SnapshottingSwift/Snapshots.swift

Is support for this something the maintainers would be interested in adding?

[swhitty]

Great idea @noahsmartin — happy to add this.

I have added this to the Package.swift in #98.