Prototype a WebExtension experimental highlighter API

[captainbrosset]

This can be done in Firefox, see the docs here: https://webextensions-experiments.readthedocs.io/en/latest/basics.html

Using this, we could prototype an API that can highlight nodes in the DOM. This way we wouldn't need to have any code here (and ugly markup in the content page) to highlight elements.
Also, I think this would generally be very useful to other addons.

This only works in pre-release versions though. So this is just a prototype.