Fix right-to-left selection not working.

[robert-ancell]

Fixes flutter/flutter#57636

[stuartmorgan-g]

I suspect that the mixture of abstract things like "forward" and direction-specific things like "left" and "right" are going to bite us when we add RTL text handling, but since this is internal we can cross that bridge when we get to it (and currently we aren't tracking text direction in the model).

[robert-ancell]

I suspect that the mixture of abstract things like "forward" and direction-specific things like "left" and "right" are going to bite us when we add RTL text handling, but since this is internal we can cross that bridge when we get to it (and currently we aren't tracking text direction in the model).

Yeah, I was concerned about that too. I originally had the methods public and then made them private so we can rethink the terminology when that complexity comes later.