QuickJS backend

[alshdavid]

Add an optional QuickJs backend with the same "userland" API ensuring all user code/extensions are compatible regardless of the backend.

fn main() {
  let runtime = JsRuntime::initialize_once(JsRuntimeOptions {
    backend: JsRuntimeBackend::QuickJs,
    ..Default::default()
  })?;
}

Also add a mqjs backend maybe?