Add WatchOS support

[roelvanderkraan]

WatchOS is currently not supported because it doesn't have UIDevice available, which is required to generate a User Agent.

/Users/builder/builds/TDmZkXJm/3/finestructure/swiftpackageindex-builder/spi-builder-workspace/Sources/SimpleAnalytics/UserAgent.swift:32:28: error: cannot find 'UIDevice' in scope
       let currentDevice = UIDevice.current
                           ^~~~~~~~

See build reports on https://swiftpackageindex.com/simpleanalytics/swift-package