Given an active region, `checkbox/toggle` should toggle checkboxes in region

[camdez]

Consider the behavior of org-toggle-checkbox:

Toggle checkbox status or (with prefix arg) checkbox presence at
point. With double prefix argument, set it to ‘[-]’, which is
considered to be an intermediate state.

If there is an active region, toggle the first checkbox in the
region and set all remaining boxes to the same status as the
first. With a prefix arg, add or remove the checkbox for all items
in the region.

If the cursor is in a headline, toggle checkboxes in the region
between this headline and the next (so not the entire subtree).

If there is no active region, just toggle the checkbox at point.