After thinking about it a bit, I've decided to allow using a multithreaded tokio executor in #spiderfire.

Although JS is single-threaded, being able to perform stuff like network requests or file reading on another thread for some additional concurrency would be nice.

Just switched it to using tokio::task::spawn_local, so the futures don't move off the main thread. If any developer wants though, they can still use tokio::task::spawn.

https://github.com/Redfire75369/spiderfire/commit/bec8f5948b4d9bda1a462039496b35af6e326904