Bug 1815969 - part 1: Redesign `EditorBase::FireClipboardEvent` r=m_kato

It does not handle critical cases properly, and it uses an out-param.
We can rewrite it with `Result`.  However, unfortunately,
`nsCopySupport::FireClipboardEvent` does not return error.  Therefore,
the root callers still need to treat the error cases as "canceled".

Differential Revision: https://phabricator.services.mozilla.com/D176737