Re-implement `.styled-text` in Tailwind:

* Simplify as much as possible and only impose rules that directly apply
  to a block of styled text
* Re-generate heading size scale using `type-scale.com` (previous tool
  is no longer extant)
* Ensure that all rules operate with `em` units (proportional to
  inherited font size)

A subsequent step would be to extract a styled-text reusable component.