feat!: Kill ScaleDetector in favour of ScaleCallbacks

[luanpotter]

Description

Kill ScaleDetector mixin and associated code from old infra in favour of ScaleCallbacks.

Minor differences:

• On panning: the old ScaleGestureRecognizer also fired scale events for a single pointer, which the zoom example exploited to pan. MultiDragScaleGestureRecognizer only fires scale once two or more pointers are down, so the example and the docs now combine ScaleCallbacks with DragCallbacks, which is the intended way to do pan + pinch in the new system and no longer needs the old PanDetector/ScaleDetector conflict workaround. The drag handler skips panning while isScaling, since a pinch emits drag events for each pointer as well.
• On trackpad pinch: ScaleGestureRecognizer handles PointerPanZoom* events via addAllowedPointerPanZoom, which is how a trackpad pinch reaches a scale recognizer. MultiDragScaleGestureRecognizer does not implement this, so nor does our new detectors; however that is an existing gap, and we can close it on followups. This is tangential to this cleanup.

Checklist

• I have followed the Contributor Guide when preparing my PR.
• I have updated/added tests for ALL new/updated/fixed functionality.
• I have updated/added relevant documentation in docs and added dartdoc comments with ///.
• I have updated/added relevant examples in examples or docs.

Breaking Change?

• Yes, this PR is a breaking change.
• No, this PR is not a breaking change.