Add #member_type field to member type Tag classes.

That is for tags: @cfg, @property, @method, @event, @css_mixin,
@css_var.  For the latter introduced also a Tag class (although
there is no corresponding @tag).

The regex for matching member types now gets constructed in TagRegistry
class, which has the central knowledge of the available member types.
Similarly exporting uses TagRegistry.member_types to loop over all
available member types (removing hardcoded member types list from
JsDuck::Class#each_member_type).