wasm omp with pthread attempt

[pca006132]

I tried porting the simpleomp implementation in ncnn (https://github.com/pca006132/manifold/tree/wasm-omp), added a simple barrier implementation, but stuck with some weird memory corruption error. I am leaving this here in case anyone is interested in doing something with this.

In general, I think the performance will probably be much worse than native even if it works, because the workers cannot perform context switching (the stacks are not accessible from wasm).