JSON Schema highlighting

[alexbainter]

Closes #159.

Adds keyword highlighting for JSON schema keywords to Highlight.js's built-in JSON language. Highlight.js themes already generally color "keywords" differently from "attributes," so we just needed to add a new "mode" to Highlight.js's JSON language definition to recognize JSON schema keywords and apply the "keyword" class to them instead of "attribute."

In this case, we're actually replacing the JSON language definition with our new JSON schema-aware one, but theoretically we could add it as an additional language instead (though in my limited testing, Highlight.js was auto-selecting the original JSON definition instead of our new one, so we'd need to sort that out).

[alexbainter]

PS, Highlight.js is not interested in accepting new contributions, though they do offer to optionally host repositories under their highlightjs GitHub organization: https://highlightjs.readthedocs.io/en/latest/language-contribution.html. I'm not sure this is quite ready for use by others due to the issue I mentioned where Highlight.js was still automatically selecting the regular JSON highlighter even when given a JSON schema file. We could certainly look into that some more and then package this up for others.

[lisad]

This seems by far the most efficient approach - if we were going to spend more work on it it might be worth sharing, but this is something anybody else could just copy.