title: How to detect tap on the scrim of the modal bottom sheet?
body: <p>I am using a modal sheet, when I tap somewhere outside the modal bottom sheet (In the transparent area), it closes the bottom sheet, so I need a callback of this tap gesture so that I can perform certain actions before closing the bottom sheet.</p>
  <p>There is property <code>isDismissible</code> in the showModalBottomSheet, which disables the tap on the scrim, but I don't want it to disable it, just need a callback so that certain actions can be performed before closing.</p>