Author a draft `Package.swift` for RNSentry

RNSentry does not currently publish a `Package.swift`. To support React Native's eventual SPM-based autolinking (tracked broadly in [https://github.com/react-native-community/discussions-and-proposals/issues/587]()) and Apple's preferred distribution model, we need a Swift Package manifest that consumers can resolve.

This issue produces a *draft* manifest. It is not expected to be end-to-end consumable until React Native's community CLI ships non-experimental SPM autolinking. The goal is to **be ready**, and to surface integration issues early.