Twenty Twenty: Replace Smooth Scroll JS implementation with `scroll-behavior` CSS property.

The JS implementation had multiple issues and did not work as expected.

This change includes an accessibility enhancement by using `prefers-reduced-motion: reduce` media query property for users that don't want motion effects. For further explanation on this media query, see MDN documentation: https://developer.mozilla.org/en-US/docs/Web/CSS/scroll-behavior#Accessibility_concerns

Props audrasjb, melchoyce, joostdevalk, Anlino, mauteri, sergiomdgomes, littlebigthing, williampatton, netweb, andraganescu, joyously, acosmin, mukesh27, hareesh-pillai.
Fixes #48763, #48551, #48866.
Built from https://develop.svn.wordpress.org/trunk@46824


git-svn-id: https://core.svn.wordpress.org/trunk@46624 1a063a9b-81f0-0310-95a4-ce76da25c4cd