Gracefully degrade from higher precision colors to lower precision

[epage]

This was split out of #5.

What if anstyle-stream provided a way to use trucolor and have it automatically adapted to Ansi256 or Ansi16, depending on what the terminal supports?

[epage]

My main concern is if we apply this too aggressively, we'll frequently be parsing ANSI escape codes that just contain what the terminal already supports, slowing things down unnecessarily.