add simple JS backend

[zamfofex]

I was told this might be desirable. A couple things to note:

• I wish to be able to publish this as a package to npm eventually. (I hope this can be allowed!)
• I have only tried this with Emscripten v3.1.57, see: Compiled code using pthreads breaks Vite bundler emscripten-core/emscripten#22394
• If there is anything I can improve, I’d love to try to!
• I set this as a draft pull request because I think the documentation could be improved, and also because I have set up some ORT‐web compatibility changes very directly, whereas they should probably be conditional.
• This only works well in browsers with WebGPU enabled (and in computers with a decent GPU), so this is currently fairly niche.

If anyone has any thoughts on whether this is warranted, I’d love to know! (Do let me know what you think!)

Some more technical details: This works by converting an Lc0 network to ONNX format and then using the ORT‐web APIs to allow Lc0 to interface with the network through JavaScript.