Bundled Themes: Twenty Nineteen horizontal rule is very narrow.

Fixes the issue by adding specific styles for the `<hr>` tag.

Props laurelfulford, nielslange, kjellr, samful, leprincenoir.
Fixes #45912.
Built from https://develop.svn.wordpress.org/trunk@48073


git-svn-id: https://core.svn.wordpress.org/trunk@47840 1a063a9b-81f0-0310-95a4-ce76da25c4cd