Fix issues with <ruby> tag

[Sub6Resources]

#178 added support for the <ruby> element, but there are a couple small issues with text scaling and line height when using the <ruby> element. These issues mostly stem from using the Transform widget, which can draw outside of its parent's boundaries.

[JeskTop]

Style could not work for <ruby> and <rt>. This is another problem.

style: {
  "ruby": Style(
      color: Colors.red,
      fontSize: FontSize(18.0)
  )
}

[Sub6Resources]

Thank you @JeskTop. We'll work to resolve those issues as well.