Add Demo Projects

[orchetect]

Proposal

Build a minimal multi-platform SwiftUI app Xcode project demonstrating all of the available features of swift-prefs.

Action Items

• Add basic @PrefsSchema demo project
• Add advanced demo for custom storage backend implementation
• Add advanced demo for custom serialization import/export format implementation (Prefs Storage Mapping Strategy (Import / Export) #9)
• Reference demo in README file