Add support for relative image paths in workshops

[johnpryan]

It might be possible to configure our markdown generator to support relative paths for image URLs. This is a pain point for authors who develop on a local machine on localhost and would like to also make the image requests to localhost too.

[johnpryan]

We might be able to provide a custom imageLinkResolver that can handle relative links correctly.

[parlough]

With https://github.com/dart-lang/dart-pad/issues/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!