Emscripten build target

Hi, I don't know if the emscripten build target is in scope of this project, or if this is an acceptable approach for this repo, but I needed it, tested with current latest zemscripten main (https://github.com/zig-gamedev/zemscripten/tree/dca717d4779423768ce4b8723f2bf8aeb14468b2): 

https://github.com/zig-gamedev/zmesh/pull/12/files