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.

Start with the linked React Native SPM autolinking proposal and the repository's existing iOS dependency layout, then author the draft Package.swift. Done means the manifest is present and integration issues are surfaced; end-to-end consumption is not required until non-experimental autolinking exists.

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