Add support for macOS 12

[eirnym]

Currently Nuit won't build on macOS 12 (with Switft 5.7).

Please, add support for earlier versions of macOS. Otherwise application distribution is very limited.

[fwcd]

Support for older macOS versions is currently not a priority, sorry. Maybe in the future if I get around to add proper availability checks etc, but currently this is largely a research project that aims to explore what's possible with the latest APIs (same reason for why I use nightly compiler features).

[eirnym]

I understand your position. I've tried to down numbers, but it won't compile as too many new swift features is used.

As I need to develop few apps for macOS and then support them, I'll switch to another library, maybe I'll return later