Fix linkDecorationHandler targeting

[philipherrmann]

What

We observed that the 3.0.3 snowplow js tracker is decorating the wrong elements.

How to reconstruct

In a situation like this

    <a href="abc">
        <img src.../>
    </a>

the sp.js tracker would add an event listener to the a-Tag, if abc matches the crossDomainLinker criterion. Now if the image is clicked. The event listener receives and event e and decorates a.target.href, but it should decorate a.currentTarget.href. Thus, the links default action will follow an undecorated link. The responsible code is here:

snowplow-javascript-tracker/libraries/browser-tracker-core/src/tracker/index.ts

Line 327 in c79a74e

let elt = evt.target as HTMLAnchorElement | HTMLAreaElement | null;

In earlier versions of the sp.js tracker the event listener ignored the incoming event, i.e. like () => { decoration; }, and then decorate the href of this, which actually equals e.currentTarget.

Newer versions of the sp.js tracker are also impacted.