[Feature request] Conditional/version-gated property definitions in @objcproperties macro

[christiangnrd]

Would prevent situations like this:

https://github.com/JuliaGPU/Metal.jl/blob/6a760a60a33b864b2c7fa3e856e38e1e6299f871/lib/mps/ndarray.jl#L52-L75

where the whole code block has to be duplicated just to add 3 definitions.

An in-macro version of this would allow for cleaner code and would allow for easier integration with the AVAILABILITY attributes once Clang header generation is supported.

This would require that either Metal.macos_version() be moved to this package, or maybe the macro take a predicate for each property definition?

[maleadt]

Implemented by #48