Add Metal support

[ViralBShah]

As the M1 GPUs are becoming popular, it would be great to add Metal support to KernelAbstractions.jl.

cc @maleadt

[bjarthur]

worth noting that at 128GB unified memory the mac studio has the largest VRAM of any vendor (that i know of). though i guess that's two chips stacked together so it might be more comparable to a multi-GPU box with high-speed interconnects.

[garymm]

I guess this should be noted in the README, which lists supported backends but does not mention Metal

[ViralBShah]

Submit a PR to the README?