Built-in themes from PlantUML

[Marco Eidinger]

Previously it was possible to include a 3rd party theme through the !include directive and that was reflected in SwiftPlantUML by includeRemoteURL option in yml configuration.

Nowadays PlantUML has !theme directive and ships with many built-in themes. See https://plantuml.com/theme for further details

The goal is to cover those built-in themes