feat: add support for real user name

[simontaurus]

add a config option to display the user's real name in the user menu. Useful in combination with Extension:Realnames

not really happy with the string replace approach - not sure if there's an option to adapt the generation of $userPageData['html-items'] directly

[alistair3149]

Probably doesn't need a config, we can check whether real name exists. If one exists, then display both.
The string replace method is fine since it has the least headache. The real name item would link to the same user page anyways. I would suggest doing the same thing, but instead replace the username with both real name and user name wrapped in divs, like this:

<li id="pt-userpage-2" class="mw-list-item">
    <a href="/User:DefinitelyNotJohn" title="Your user page [alt-shift-.]" accesskey=".">
        <div class="pt-userpage-realname">John Doe</div>
        <div class="pt-userpage-username">DefinitelyNotJohn</div>
    </a>
</li>

And with some CSS, it should turn into this:

My MW setup is broken at the moment so I can't test PHP changes, but I can help with the CSS part if needed.

[simontaurus]

thanks for solving this and sorry I didn't manage to update the pull request according to your comments in the meantime.