Sunstone v0.27.0

Added

• D2 diagrams. A ```d2 code block renders as a diagram in editing
  and read mode, the web viewer and the PDF export. It is drawn in the
  app's colours and follows a theme switch; colours you set in the
  diagram are kept. D2 loads only when a Concept contains a D2 block,
  and external icons are never fetched.

Fixed

• The print preview uses the light colours it prints with, also when
  the app is in dark mode.