Provide a way to ask for highlighting that fails if deps are missing

[dontlaugh]

Somehow the caller should be able to ask the library for a highlighted block, without a fallback to unhighlighted.

[finanalyst]

The problem is that to have highlighting at all requires a Node dependency, which is large and time consuming. It took me a long time to figure it our because I never use Node for anything else.

Once Liz has got her RakuAST highlighter working, I'll make this happen.

[jubilatious1]

Maybe related:

If you go to the ReadMe (Installation) paragraph:

https://github.com/finanalyst/raku-pod-render

and click on "Highlighting":

https://github.com/finanalyst/raku-pod-render/blob/master/Highlighting.md

you get a '404 page-not-found' error.

[finanalyst]

@jubilatious1 Thanks for the catch. The problem is that I generate my .md files automatically from .rakudoc files of the same name, and the links are not the same for github and a local version. Most of my free time at the moment is rewriting this module for RakuAST, so I hope that this module will eventually be redundant.
Also the highlighting feature really needs to change.