Add extern feature #20
Description
A no_std backend which supports externally added async workers for concurrency. Test with embassy and maybe tokio.
Users will be expected to spawn their own tasks on the executor, which then call some sort of async forte worker. One worker will probably be sufficient. All async work on the pool, spawn and so on will then be executed by that worker, allowing the Forte to become a shim over whatever async runtime the user wants to install it in.
Basically, this should allow bevy (or anything built on forte) to run kinda-ok-ish on any async executor.