Use CompositedTransformTarget and CompositedTransformFollower when possible

[matthew-carroll]

follow_the_leader forces users to choose between Flutter's built-in composited target/follower widgets and our Leader and Follower widgets. This can sometimes be a choice based on performance. Flutter's built-in widgets are more performant because they're much less capable.

Rather than force users to choose us or them, adjust Leader and Follower to use Flutter's standard composited transform Layers when the user doesn't use a FollowerAligner or FollowerBoundary, and therefore the user doesn't need what we provide.