Add SaneDev implementation

[Jupi007]

I have created a SaneDev implementation for my Flutter UI project, and I will import it here.
This allows us to test the API without needing a real scanner device by "emulating" it.

Btw, this is why I want to decouple the isolate implementation, because you can swap the SaneDev instead of Sane in the constructor and it will work as it is. This is very useful for testing.

[Craftplacer]

What speaks against making Sane just the interface itself? And it contains factories for implementations like .sync or .net, or the isolated one?