Fix crash in ClassFormatter#inject_formatter_to_tags.
Turn @static into builtin Tag class.

Lots of changes, but they're all pretty much the same - replacing
foo[:meta][:static] with foo[:static].
Turn @since & @new to builtin Tag classes.

For now I'm leaving out the tooltip text implementation for
the @new tag - needs some further refactoring.
Turn @aside to builtin tag class.

For now the injection of @assets is implemented as specific to @aside
tag.  Needs cleaning up in the future.  But now finally all the old
meta-tags have been replaced with new builtin tags.
Initialize Tag classes in alphabetic order.
Exclude abstract BooleanTag from lists of concrete tags.