No syntax highlighting for shell hover contents

[christinaforney]

When visiting https://gitlab.com/freedesktop-sdk/freedesktop-sdk/blob/master/utils/find-rpath.sh

[felixfbecker]

This is the graceful handling :) The hover will still render, it will fall back to plain text and log a warning so we are informed a language is not supported (which I would want to keep).

The issue I see here is that we don't recognize "shell". Unless you disagree, I'll rename the issue accordingly

[felixfbecker]

This just needs to register the shell highlight.js highlighter under the shell name

[eseliger]

I think 'shell' doesn't exist in highlight.js. bash does though, so maybe the mapping is wrong?