Async Encoding

[DjDeveloperr]

Just need to offload encoding to a separate thread and call with non blocking: true. It'll be useful for applications which handle concurrent requests and render images for them.

Also see: #27

[lino-levan]

Implementation-wise, this should be done with web workers right?

If so, we should probably mention that this breaks compatibility with deno compile

[DjDeveloperr]

Nope, workers aren't needed. We just need to add another FFI symbol definition that refers to same encoding symbol but with nonblocking: true.