IE 11 support drop

[bbecquet]

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

• 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