Add UI for notes, warnings, and tips

[johnpryan]

Workshops often need to tell the user to take an action:

It could be helpful to add some special syntax for this:

We could add some special CSS classes, and authors could add some HTML to their markdown files:

<div class="dartpad-workshop-note">
Note goes _here_.
</div>

[parlough]

With #2808, support for workshops was dropped, so I'll close this as obsolete. If anyone reading this desires a feature similar to this in DartPad or a new form of workshops, please discuss that in a new issue. Thanks!