Bundled Themes: Twenty Twenty Social Icons Filter.

Adds a Social Icons Filter to Twenty Twenty that allows for new icons to be added to the theme by filter.

Props JarretC, nielslange, acosmin, desrosj.
Fixes #48713.
Built from https://develop.svn.wordpress.org/trunk@48180


git-svn-id: https://core.svn.wordpress.org/trunk@47949 1a063a9b-81f0-0310-95a4-ce76da25c4cd