Feature: Add SwiftUI ViewModifier for shake animation

[haaakon]

Summary

• Adds ShakeEffect — a GeometryEffect that drives a sine-wave horizontal translation, matching the UIKit version's back-and-forth oscillation behaviour.
• Adds ShakeModifier — a ViewModifier wrapping ShakeEffect. Increment its trigger: Int to start a new shake. Accepts an optional accessibilityAnnouncement that is posted via UIAccessibility when VoiceOver is active (mirrors postAccessabilityNotification in the UIKit version).
• Adds a .shake(trigger:accessibilityAnnouncement:) convenience extension on View so usage stays a single line.
• Adds Package.swift for Swift Package Manager distribution alongside the existing CocoaPods podspec.
• Updates README.md with a full SwiftUI usage section (basic, VoiceOver, and full button-driven example).

Usage

// Basic – increment trigger to shake
TextField("Username", text: $username)
    .shake(trigger: errorCount)

// With VoiceOver announcement
TextField("Username", text: $username)
    .shake(trigger: errorCount,
           accessibilityAnnouncement: "Username field is required")

Test plan

• Build the package with swift build (requires Xcode 13+ / swift-tools-version 5.5)
• Add SingleLineShakeAnimation via SPM in a SwiftUI project and verify .shake(trigger:) compiles
• Tap a button that increments errorCount and confirm the target view shakes visually
• Enable VoiceOver, increment trigger, confirm announcement is read and visual shake still fires
• Verify the existing UIKit UIView.shake(_:) API is unaffected