feat: add exit presence animations

[NikhilRW]

Summary

• Add EasePresence to retain keyed EaseView children until their exit animation completes.
• Add the exit prop, skip native forwarding, and warn about missing keys or looping exit transitions.
• Update the exit demo, docs, migration skill, and tests.

Checks

• yarn test src/tests/EasePresence.test.tsx --runInBand — passed (5 tests).
• yarn lint:ts and yarn lint:ts:example — passed.
• Scoped ESLint and Prettier checks — passed.
• The full test suite has one failure in EaseView.test.tsx: React Native's Jest Text mock throws while rendering its existing children test.

Closes #46

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[NikhilRW]

I am closing because I think I vibe coded it too much