Add syntax highlighting for lisp examples

Texinfo has an API to customize the items that are placed in the HTML HEAD section of the generated HTML file(s).

It would be pretty cool if we could add highlighting of the Lisp code in all the examples in all of the HTML versions of the manuals.

Maxima recently added this to their user manual. For example, see the highlighting of the examples at https://maxima.common-lisp.dev/docs/Special-Functions.html#Gamma-and-Factorial-Functions