MarkdownDialectGitHub now includes this feature, so *emph* is italicized
and _emph_ is underlined. This is a better fit for QTextDocument capabilities;
until now, _underlined_ markdown could be read, but would be rendered with
italics, because in CommonMark, *emphasis* and _emphasis_ are the same.
But QTextMarkdownWriter already writes underlining and italics distinctly
in this way.
[ChangeLog][QtGui][Text] By default (with MarkdownDialectGitHub), markdown
_underline_ and *italic* text styles are now distinct.
Fixes: QTBUG-84429
Change-Id: Ifc6defa4852abe831949baa4ce28bae5f1a82265
Reviewed-by: Volker Hilsheimer <volker.hilsheimer@qt.io>
f5c7799f59