Fix focusing last line when using data-trim

[toby-allsopp]

The last line was not being wrapped in a when using data-trim because the
trailing newline was removed. Add a trailing newline.

[bnjmnt4n]

Thanks for the PR!

[bnjmnt4n]

I've come to realise that previous behaviour was buggy: when parsing something like

<pre><code>// abc
// abc
</code></pre>

The third line was treated as non-existent, which does not follow the browsers' display of a third line.
I'll change the behavior to a more semantic one later.

Edit: Seems like I've misunderstood the behaviour, will take some time to review and update.