Implement pinch to zoom

On mobile devices, it's a bit hard to crop the image by moving the circle anchors. We should implement "pinch to zoom" or some other functionality to make this easier for touch users.

Start by locating the touch interaction code for the image-crop custom element and reviewing how mobile users currently move the circle anchors. Define the intended pinch-to-zoom behavior and verify that cropping remains accurate during the gesture. The issue names no files or tests, so the relevant interaction coverage will need to be located first.

Written by the indexing model from the issue text.