Add onLongPressDown, onLongPressCancel (#81260)

This adds support for GestureDetector.onLongPressDown and
GestureDetector.onLongPressCancel, allowing callers to register
interest in the initial pointer contact that might turn into
a long-press (and the associated cancel event if the gesture
loses).

packages/flutter/lib/src/gestures/tap.dart

@@ -394,6 +394,7 @@ class TapGestureRecognizer extends BaseTapGestureRecognizer {
   /// See also:
   ///
   ///  * [kPrimaryButton], the button this callback responds to.
+  ///  * [onSecondaryTap], a similar callback but for a secondary button.
   ///  * [onTapUp], which has the same timing but with details.
   ///  * [GestureDetector.onTap], which exposes this callback.
   GestureTapCallback? onTap;

packages/flutter/test/rendering/editable_gesture_test.dart

@@ -41,6 +41,7 @@ void main() {
     editable.attach(owner);
     // This should register pointer into GestureBinding.instance.pointerRouter.
     editable.handleEvent(const PointerDownEvent(), BoxHitTestEntry(editable, const Offset(10,10)));
+    GestureBinding.instance!.pointerRouter.route(const PointerDownEvent());
     expect(spy.routeCount, greaterThan(0));
     editable.detach();
     expect(spy.routeCount, 0);

packages/flutter/test/widgets/scrollbar_test.dart

@@ -922,6 +922,7 @@ void main() {
     // Move back to reset.
     await dragScrollbarGesture.moveBy(const Offset(0.0, -scrollAmount));
     await tester.pumpAndSettle();
+    await dragScrollbarGesture.up();
     expect(scrollController.offset, 0.0);
     expect(
       find.byType(RawScrollbar),