Add SaneDev implementation

[Jupi007]

This implementation allows us to test the library API without a real scanner connected.
The isolated implementation can switch between real and fake:

-final sane = SaneIsolate(sane: Sane());
+final sane = SaneIsolate(sane: SaneDev());

Every method is delayed by 1 second, except init() and exit().

Fixes #15

[Jupi007]

I plan to improve this SaneDev class by adding multiple devices each one having a different behavior (like throw fake error at some step), this way we can easily test and develop the frontend app.

[Craftplacer]

Looks alright, albeit with many delays.

[Jupi007]

I need to bench the real sane implementation to how long it takes, and adapt the delay.
But for now 1s is okay.