Proposal: Replace modernizr with modernizr-esm

[roblarsen]

This builds off of #2637

This is a bit out there since modernizr-esm has like 20 stars, but... the idea of doing a custom element example and then testing against a window.customelement global is kind of a bummer.

This is the way it should look in 2021 and the main Modernizr project is on life-support. Yet, the need still exists.

[roblarsen]

👋 @niksy