Add async versions for NbtFile Read/Write methods

[mstefarov]

No description provided.

[eamonwoortman]

Hi @mstefarov,
You're probably not maintaining this project any more, but I was wondering how you would go about changing the ReadWrite methods to async.
I've tried introducing the tasks into every different aspect of it (all the way to the NbtBinaryWriter/Reader), but I'm experiencing some issues already.

Thanks for this fork though, it works perfectly!