[JS] Move from embind to manual bindings

[NickGerleman]

embind is convenient but not ideal

1. Its structure makes it hard to implement garbage collection vs finalization registry because we can’t differentiate JS refererence Vs native pointer
2. It bloats the binary/bundle and adds required runtime support. Goes in the opposite direction of being able to generate portable wasm targeting only WASI.
3. Our patching of its interface is hard to reason about or add to.

We should instead add the code for the object representation and have it call Yoga C APIs directly, like the JNI bindings.

[NickGerleman]

This isn’t something I’m going to prioritize, but might be worth picking up as a next step in evolving the bindings, especially if standalone wasm es modules become well supported.