Add Swift Package Manager opt-ins for RN_STRICT_API and RN_ALLOW_FRAMEWORKS (#59043)

[coado]

Summary:

Let iOS apps that use the Swift Package Manager setup enforce React Native's C++ stable API, and control its "for frameworks" warning, matching the CocoaPods and Android opt-ins. Two env vars are read by npx react-native spm add / update:

• RN_STRICT_API=1 defines the RN_STRICT_API macro, which arms the react/cxxstableapi guards.
• RN_ALLOW_FRAMEWORKS=1 defines the RN_ALLOW_FRAMEWORKS macro, which silences their "for frameworks" warning.

Each variable is named after the macro it defines, as in the CocoaPods opt-in.

Each opted-in macro is defined for the app's own code:

• the app target (GCC_PREPROCESSOR_DEFINITIONS, seeded with $(inherited)),
• the packages the autolinker generates for the app's local spm.modules,
• the app codegen package (ReactCodegen, ReactAppDependencyProvider).

Generated code never warns: the codegen package always defines RN_ALLOW_FRAMEWORKS, because nobody can act on a warning in generated code and libraries are held to the API in their own builds.

Under SwiftPM every library builds from its own Package.swift (shipped upstream or created with spm scaffold), so a library declares RN_ALLOW_FRAMEWORKS there, and the app's opt-ins do not reach libraries. React Native itself ships prebuilt, so none of its code is compiled with these macros.

The Xcode build phase runs spm sync without the shell environment, so add/update record the chosen macros in .spm-injected.json and every later sync regenerates the packages from that record. As with pod install, each add/update re-reads the variables: running update without one turns its macro off, and deinit removes the app-target defines along with the rest of the injection.

The SwiftPM guide (scripts/spm/__docs__/spm-scripts.md) documents the opt-ins: what they define, that libraries are only checked when they define the macros in their own Package.swift, and how the choice is remembered for Xcode builds.

The opt-ins are one list (CXX_API_DEFINES in spm-utils.js) passed through as macro names, so adding another opt-in is a one-line change.

Changelog:
[iOS][Added] - Add RN_STRICT_API and RN_ALLOW_FRAMEWORKS opt-ins for the Swift Package Manager setup to enforce React Native's C++ stable API in the app's native code

Differential Revision: D124985657

[meta-codesync]

@coado has exported this pull request. If you are a Meta employee, you can view the originating Diff in D124985657.