feat: add shadow properties (iOS) and elevation (Android)

[janicduplessis]

Summary

React Native's shadow API (shadowOpacity, shadowRadius, shadowColor, shadowOffset) is iOS-only and uses CALayer properties, while Android uses elevation for material shadows. Neither is currently animatable through EaseView.

This PR adds native animation support for both platforms following the existing per-property pattern:

• iOS: shadowOpacity, shadowRadius, shadowColor, shadowOffsetX, shadowOffsetY — animated via Core Animation key-path animations on the corresponding CALayer properties. Shadow offset X/Y are animated together as a single shadowOffset CGSize animation.
• Android: elevation — animated via ObjectAnimator (timing) with spring falling back to timing, matching the borderRadius pattern.

All six properties get their own bitmask flag so they can coexist with style-managed values. A new shadow transition category allows per-category animation config for all shadow/elevation properties.

Test Plan

• yarn format:write && yarn lint && yarn test — all 71 tests pass, including 15 new tests covering shadow/elevation prop resolution, bitmask flags, initialAnimate, shadowColor, and style conflict stripping.
• Native behavior should be verified by running the example app on iOS simulator and Android emulator with shadow/elevation in animate:

<EaseView
  animate={{
    shadowOpacity: 0.5,
    shadowRadius: 10,
    shadowOffsetY: 4,
    elevation: 8,
  }}
  transition={{ shadow: { type: 'spring', damping: 15 } }}
/>