Revert strong regex changes from #644 for legacy IAB implementation (#688)

[Crozzers]

This PR addresses #688, which is due to the changed rules around strong/em introduced in #665.
In that PR, alot of previously valid strong/em examples became invalid.

In theory, users could do the following to restore the existing behaviour:

class ForceOldIAB(markdown2.ItalicAndBoldProcessor):
    name = 'force-old-iab'
    order = (markdown2.Stage.ITALIC_AND_BOLD,), (markdown2.Stage.ITALIC_AND_BOLD,)

    def sub(self, match):
        syntax = match.group(1)
        tag = 'strong' if len(syntax) == 2 else 'em'
        return f'<{tag}>{match.group(2)}</{tag}>'

ForceOldIAB.register()

markdown2.markdown(text, extras=['force-old-iab']

But #644 changed the strong regex before being superseded by #665 in the same release cycle.

Since that change was never actually released, it may make sense to revert it so that users can opt back into the old IAB behaviour for compatibility