Copy the `WebRuntime` from wasmer.sh into the `wasmer-js` repository

[Michael Bryan]

With the switch from wasmer-wasi to wasmer-wasix, we now need to provide a wasmer_wasix::runtime::Runtime implementation for things like network access and task management.

We've already implemented this for browser environments as part of wasmer.sh, so we should be able to reuse a lot of code from there:

https://github.com/wasmerio/wasmer/blob/3dbe13dfd611a440a96faa137fbbafc5cbb1f20f/lib/wasi-web/src/runtime.rs#L49-L107

[Michael Bryan]

Implemented in #328.