function cacheShadowRect(dz) {
    const shadowElIndex = Array.from(dz.children).findIndex(child => child.getAttribute(SHADOW_ELEMENT_ATTRIBUTE_NAME));
    if (shadowElIndex >= 0) {
        if (!dzToShadowIndexToRect.has(dz)) {
            dzToShadowIndexToRect.set(dz, new Map());
        }
        dzToShadowIndexToRect.get(dz).set(shadowElIndex, getAbsoluteRectNoTransforms(dz.children[shadowElIndex]));
        return shadowElIndex;
    }
    return undefined;
}
Caches the coordinates of the shadow element when it's in a certain index in a certain dropzone.
Helpful in order to determine "would be index" more effectively
@param {HTMLElement} dz
@return {number} - the shadow element index