Add extension package for StaticArrays

[trahflow]

The motivation for this PR is to @device_override a particular throw-method in StaticArrays to make it GPU-compatible.
This is done like in src/device/quirks.jl, but placed in a new extension package for StaticArrays as suggested by @maleadt here.

@maleadt do I understand you right that the idea is to also make KernelAbstractions a weak dependency? This would have to be done as part of this PR then I guess? Otherwise I'm unsure how having StaticArrays as both, a direct dependency (due to the fact it's used in the KA backend code) and an extension package (as introduced here) would work out.
For now I'm directly including the code, albeit it still lives in ext/.

The test (actual test and the file layout) is just a suggestion. I'm not familiar with CUDA.jl's test setup, and I always have a bit of an issue with code loading when dev-installing a package that has extension packages.

[maleadt]

Thanks for the PR!

@maleadt do I understand you right that the idea is to also make KernelAbstractions a weak dependency?

KernelAbstractions.jl can remain a normal dependency, as that would be a much more invasive change. I'm hoping to get rid of the StaticArrays dep, which wouldn't necessarily demote KA.jl to a weak dep.

[trahflow]

Thanks for the review!

I've moved the test and improved the error message a bit (mentioning conversion to StaticArray).