jsCoq 0.16.0 "Paris-bound"

• Update to Coq 8.16.0. (@corwin-of-amber, @ejgallego)
• Now Coq loads plugins using findlib, but we don't yet support that;
  most plugins can still load in legacy mode.
• Port the JS codebase to ES modules (@ejgallego , @corwin-of-amber, #276)
• Add a quick help screen in the UI (@corwin-of-amber, #290)