Add a SwiftUI wrapper for PhoneNumberTextField

[AndrewSB]

What changed

• add a public PhoneNumberTextFieldRepresentable with a two-way String binding
• expose a configuration closure for the underlying PhoneNumberTextField
• document SwiftUI usage

Why

PhoneNumberKitUI exposes UIKit components but currently leaves every SwiftUI consumer to maintain its own UIViewRepresentable wrapper. This moves that bridge into the package while retaining access to the full UIKit configuration surface.

Closes #2.

Validation

• xcodebuild -scheme PhoneNumberKitUI -destination 'generic/platform=iOS Simulator' build