[Refactor] Reduce code duplication for iOS 14 support

Changelog

• Replaced iOS14CompatibilityAlertView with a more general View+iOS14Compatibility.swift
• Added modifiers mimicking SwiftUI 3 alert and refreshable to reduce code duplication
  • Unfortunately, we don't have access to SwiftUI.Button's internal workings, so we have to introduce _Button
  • However, we can still use Button to construct either SwiftUI.Button or _Button depending on the arguments

What happens when we drop iOS 14 support?

1. Delete View+iOS14Compatibility.swift
2. Done

Testing

Manually tested on iOS 14.5 and iOS 15 simulators for all alerts.

Known Issues

• Pull to refresh refreshable remains unimplemented for iOS 14