highlight.js: add support for dark mode in view source

This MR picks up the new upstream stylesheets nnfx.css and nnfx-dark.css from highlight.js.

nnfx is specifically tailored to highlight html and xml, as well as css and JavaScript that is embedded in html content. This provides better contrast and readability, relative to the default highlight.js stylesheet.

In addition, this MR introduces support for GTK dark mode in view source. Currently, epiphany view source does not adjust its style for dark mode. Now, a specific dark mode sheet is used. Example normal and dark mode view source:

GTK_THEME=Adwaita

GTK_THEME=Adwaita:dark

needs !703 (merged)