v0.3.0-int.8

• Dropped support for .NET Standard 2.0. The library will only target .NET 8.0+ from now on.
• Added nullable reference type annotations for the library API.
• Introduced a visitor (base) class PlainTextNodeRenderer to replace the flawed NodePlainTextFormatter delegate. (#25)
  • This makes it possible to tweak the plain-text conversion behavior (e.g. customize how a template should be rendered) while keeping the rest of the existing behavior unchanged by deriving from the PlainTextNodeRenderer class.
  • A usage example can be found in the UT method CustomPlainTextNodeRendererTest1.