Move the web runtime into wasmer-wasix

[Michael-F-Bryan]

WARNING: Do not merge this PR!

This PR requires #4115 to be merged first. Once that is done, we can rebase and switch the target branch back to master.

[syrusakbary]

This PR requires #4112 to be merged first

#4112 was superseded by #4115. Did you mean #4115 ?

[zees-dev]

Would like to use web runtime when building WasiFunctionEnv - currently cannot seem to do so since the code panics in the build_init function:

let runtime = self.runtime.unwrap_or_else(|| {
    #[cfg(feature = "sys-thread")]
    {
        Arc::new(PluggableRuntime::new(Arc::new(crate::runtime::task_manager::tokio::TokioTaskManager::default())))
    }

    #[cfg(not(feature = "sys-thread"))]
    {
        panic!("this build does not support a default runtime - specify one with WasiEnvBuilder::runtime()");
    }
});

^ could we instantiate the WebRuntime here under the #[cfg(feature = "js")] flag?

[Michael-F-Bryan]

This functionality has now been built into the @wasmer/sdk JavaScript package in the wasmerio/wasmer-js repo.