allow HTML5 element `wbr`

[flavorjones]

This PR introduces some changes:

• deprecate HTML5::SafeList::VOID_ELEMENTS which is not a canonical set of void elements
• remove unallowed elements from VOID_ELEMENTS
• remove elements that libxml2 doesn't actually treat as void from VOID_ELEMENTS
• allow wbr element

See #212 for background. See also https://developer.mozilla.org/en-US/docs/Glossary/Empty_element

[flavorjones]

Released in v2.11.0