Refactored color_style() and no_style() to improve testability. Refs #23663.

This includes the following improvements:

- The type of the style object is now called 'Style' rather than 'dummy'.
- The new make_style() function allows generating a Style object directly
  from a config string. Before the only way to get a style object was
  through the environ and it also required that the terminal supported
  colors which isn't necessarily the case when testing.
- The output of no_style() is now cached with @lru_cache.
- The output of no_style() now has the same set of attributes as the
  other Style objects. Previously it allowed anything to pass through
  with __getattr__.