[Pitch] Generalize `conformance` macros as `extension` macros

[gwendal.roue (Gwendal Roué)]

OK, so those conformances live at another level. Good to hear!

I asked because those reports about macros that don't work with nested types were suspicious:

• @Observable doesn't work for classes defined inside an extension
• Helge Heß: "This BTW seems to happen if I nest #SwiftData `@m…" - Mastodon.