Allow custom ResultWriter implementation

[sergeidyga]

Hi!

I need to create a custom ResultWriter implementation to output assertion subjects as JSON strings (ideally, enriching the ouput with pointers to the failed properties). However, the current library architecture poses challenges for achieving this. While there are existing ResultWriter implementations like AnsiColorResultWriter and MarkdownResultWriter, they are not directly usable due to visibility and usage restrictions.

The problem lies in the fact that ResultWriter is internally defined within the AssertionStrategy class, but it's not part of its constructor. Additionally, AssertionStrategy is part of the AssertionBuilder class constructor, but that does not help since AssertionBuilder is also an internal class.

interface DescribeableBuilder
    \
  internal class AssertionBuilder(AssertionStrategy)
        \
     internal class AssertionStrategy
            \
          interface ResultWriter  

This design makes it difficult to extend ResultWriter functionality without creating custom versions of both AssertionBuilder and AssertionStrategy, which is a substantial task.

I seek guidance on the best approach to tackle this issue. Specifically, I'm interested in contributing to make the ResultWriter customizable. This enhancement could benefit both my use case and the wider user community facing similar needs.

Thank you for your assistance :)

Best regards,
Sergei