{core, mach}: provide a C ABI which allows using Mach from other languages

[Stephen Gutekanst]

This will be a separate Zig project github.com/hexops/libmach which exposes mach core and engine APIs using a nice, canonical C API. It should be possible to use just libmachcore, while using libmachengine will imply you want to use a 'standard' set of ECS modules