Move CSS type parsing skipping logic out of Tag classes.

Handle it at the Format::Class level again - but this time do it through
the Format::Doc#skip_type_parsing setter method.

Because the concrete CSS member types are hard-coded anyway it's better
to have this hard-coded logic in one spot, not scattered around several
Tag classes.
Don't skip type parsing when doing export.

Why are we skipping it on that case?  Possibly to avoid creating
the :html_type field, but currently we are creating it anyway, which
defeats the whole purpose of skipping the parsing, plus we lose warnings
of bad typedefs.

Now we get the type warnings also when doing export. And the code
is simpler. Win-win.