Add a requirejs shim for highlight.js

Before this, highlighting code in markdown cells was broken since hljs wasn't available.

A better solution might be to build the highlight.js amd module (i.e., build the highlight.js module with `--target=amd`) so that we don't need the shim.

Fixes #6187
this should probably be done in `IPython/html/templates/page.html` where the rest of the shim are.