Create adapter for Nunjucks templating engine

[CosAnca]

Is your feature request related to a problem? Please describe.
Even though there is documentation about how to create custom adapters for UIengine, I haven't really seen this happening. People would either use one of the built in ones if is part of their requirements or look for a different tool.

Describe the solution you'd like
As Nunjucks seems to be a very popular templating engine recently especially in the JAMstack world, adding an adapter to UIengine could have the potential of drawing more users and solve many of the workarounds some users are doing in order to quickly scaffold a style guide / design system / pattern library for their projects.