The nested reference click handler relied on the sup having a single
child 'a' that was always the target. If MediaWiki:Cite_reference_link is edited
this is not always true - for example a `span` wrapping [ and ]
can also be the child of the sup (and a child of the link) which
results in the link being followed.

The click handler is updated to always run on the `sup` element
using querySelector to access the single link element.

The link default behavior is suppressed separately via another
event.

Bug: T258096
Change-Id: I0d5bb398e99465ade357743dd3ff34ae551243c5