Support browsers from before 2020 (#4579)

* Support browsers from before 2020

As reported in #2825, #2826 and #3051, almost everything works in older browsers. This setting here prevented me from updating and I think it is an obvious enhancement to not restrict to browsers from 2020+. There should not be any measurable downsides of this change since es6 and es2020 are pretty similar with only minor differences.

* Include lib (polyfills) for <es2020 targets

* Assume all modern dom features despite es6 syntax

* Add modern dom iterators to es6 environment

Co-authored-by: Joe Previte <jjprevite@gmail.com>
Co-authored-by: Asher <ash@coder.com>