Explore static library support (for e.g. iOS)

[alexrp]

This isn't terribly useful right now, but will become useful once the .NET SDK gains full support for Apple platforms like iOS and watchOS that only support static linking.

This will require significant work in Zig, especially for cross-compilation.

See: ziglang/zig#5145

[alexrp]

As far as how this should work from the user perspective, I think there should just be an MSBuild property that informs the SDK which kind of library to build. NativeAOT has already introduced the NativeLib property for this; the name is a bit unfortunate, but there is probably value in standardizing on it. It accepts Shared (default) and Static. We might extend it to also accept Both.