Mozilla bug 1824121 - Implement the <search> element r=hsivonen,Jamie,emilio

Implemented the new <search> HTML element.
All WPT tests for it now pass (except one for iso-8859-8, not done generally).

A11y role uses just landmark as recommended instead of a new search role (for now).

Co-authored-by: Henri Sivonen <hsivonen@mozilla.com>

Differential Revision: https://phabricator.services.mozilla.com/D176967

Author: CanadaHonk