We can relax about `GetElementOrParentElement*()` because they just return
`nullptr` when selection anchor is a `Document` node.
Additionally, this patch renames the internal APIs to the names similar to
modern DOM API.
Finally, this adds automated test for
`nsIHTMLEditor.getElementOrParentByTagName()`.
Differential Revision: https://phabricator.services.mozilla.com/D70152
--HG--
extra : moz-landing-system : lando
