Update the SwiftPM docs to accommodate experimental and upcoming features

[heckj]

Description

The earliest proposal is https://github.com/swiftlang/swift-evolution/blob/main/proposals/0362-piecemeal-future-features.md, landed in Swift 5.8 - which doesn't appear to be reflected in SwiftPM's documentation today.

There's ongoing work with a migrate command, with an evolution proposal that hasn't been fully worked out, but is in the main branch, and is referenced in the migration guide: https://github.com/swiftlang/swift-migration-guide/blob/main/Guide.docc/FeatureMigration.md

This may be more relevant as that migrate command/feature comes to fruition, and should ideally overlap with lists of what you can enable for experimental or upcoming features, sourced from the compiler itself and aligned releases.

Expected behavior

No response

Actual behavior

No response

Steps to reproduce

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Swift Package Manager version/commit hash

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Swift & OS version (output of swift --version && uname -a)

No response

[heckj]

related articles in the Migration guide:

• https://www.swift.org/migration/documentation/swift-6-concurrency-migration-guide/featuremigration

• https://docs.swift.org/compiler/documentation/diagnostics/upcoming-language-features/

  • https://docs.swift.org/compiler/documentation/diagnostics/existential-any
  • https://docs.swift.org/compiler/documentation/diagnostics/member-import-visibility
  • https://docs.swift.org/compiler/documentation/diagnostics/nonisolated-nonsending-by-default

• https://www.swift.org/blog/using-upcoming-feature-flags/ (blog from 2023, now a bit out of date)