add a toggle

[NovemLinguae]

Requested by Bbb23 at https://en.wikipedia.org/wiki/User_talk:Novem_Linguae/Scripts/UserHighlighterSimple#Toggle

Add a toggle to turn UserHighlighterSimple on and off.

Would probably add a link to the left menu or the more menu.

Would probably need to store the state somewhere. Maybe LocalSettings or mw preferences.

Would put this behind a feature flag such as window.userHighlighterSimpleShowToggle = true;