feat: initial release of FloatingMenuButton SwiftUI package (#1)

* feat: initial release of FloatingMenuButton SwiftUI package

- add core FloatingMenuButton view with expandable menu
- add FloatingMenuItem model and item row/card views
- include Swift Package manifest
- add demo example app
- add README with preview, badges, API, install steps, and next steps
- include screenshots and demo GIF assets
- add support to custom image