feat(ext/ffi): Non-blocking FFI

[littledivy]

Adds support for FFI calls that do not block Deno's event loop. Useful for many cases eg: USB transfers, etc.

const library = Deno.dlopen(libPath, {
  // Set `nonblocking: true` to make the call async
  "sleep": { parameters: "u64", result: "void", nonblocking: true }
});

// Runs the `sleep` call in a dedicated
// blocking thread and resolve when its done
library.sleep(10000).then(() => console.log("Good morning!"))
// Do stuff

[ry]

Seems reasonable and well implemented.

[eliassjogreen]

LGTM

[piscisaureus]

LGTM