IE 11 support drop

[bbecquet]

Checklist of technical simplifications and improvements after support of IE11 has been dropped:

• [x] Remove "Explorer 11" from babel targets
• [ ] CSS vars
• [ ] Simplify conditional classList.add/remove by replacing with 2-args toggle
• [ ] Simplify array and string lookups with includes
• [ ] Simplify custom events creation
• [ ] Remove url-search-params-polyfill from dependencies
• [ ] Use IntersectionObserver instead of some costly scroll listeners
• [ ] Use hover media query to check if device supports mouseover interaction

To be completed.

[G-Ray]

-Array.includes() would be available

[amatissart]

• [ ] Remove url-search-params-polyfill from dependencies