Add support for from: modifier on intersect events

[JanRomero]

This would allow you to do infinite scroll with better locality of behaviour, by putting the attribs on the list itself:

<ul id="tweets"
    hx-get="/api/moretweetspls/"
    hx-swap="beforeend"
    hx-include="#tweets li:last-child"
    hx-trigger="intersect from:footer"
>

    <div class="tweet">
        <input hidden name="timestamp" value="123456789"/>
    </div>

    <div class="tweet">
        <input hidden name="timestamp" value="123456788"/>
    </div>

</ul>

<footer>mAdE wItH &#10084; In dUdErStaDt</footer>

Testing

A new test is included in this PR. Unfortunately it's manual. The other tests pass.
I've also added a line about the modifier to the docs.

Checklist

• I have read the contribution guidelines
• I have targeted this PR against the correct branch (master for website changes, dev for
  source changes)
• This is either a bugfix, a documentation update, or a new feature that has been explicitly
  approved via an issue
• I ran the test suite locally (npm run test) and verified that it succeeded

thx