Have more granular change detection for platforms

[brettcannon]

Right now, https://github.com/python/cpython/blob/main/.github/workflows/reusable-context.yml whether to run:

• All tests
• Build docs
• Windows tests
• Windows MSI
• Fuzzing

This could probably be expanded to cover platform-specific files so that when you edit only e.g. WASI-specific files you only run CI for WASI. The platforms we could cover are:

• macOS
• iOS
• Android
• Emscripten
• WASI

Namely every platform that has a dedicated directory where if only files in those directories are edited you could skip all other CI tests.

Linked PRs

• gh-142278: Add granular change detection for platforms in CI #142350
• [3.14] gh-142278: Add granular change detection for platforms in CI (GH-142350) #142537