v2.7.0 — Diagrams in your app's font

Diagram labels can now be set in the font the rest of your app uses. A new
DiagramTheme.fontFamily takes a family name, and every label is measured
and drawn in that family, so boxes still fit the text. At the default (nil,
the system font), output is byte-identical to 2.6.0. Additive: no existing
call changes.

• DiagramTheme.fontFamily sets and measures every label in a chosen
  family (DiagramTheme(prefersDark: false, fontFamily: "Inter")), for the
  raster renders (MermaidView, image, pdfData, attachmentString). Body
  text takes the family's regular face where it has no medium, so text never
  draws wider than it was measured. A family the device doesn't have is the
  system font, byte for byte, on Apple platforms and Linux. It's part of the
  theme's cache fingerprint. scene/svg stay in the system font. Additive;
  the default is unchanged. (#77)

Thanks to @hipstersmoothie, who built this so chat apps can draw diagrams in
the interface font their users pick.