Improvements to render style support and selection

[AnonymouX47]

• Introduces render style categories.
  • Adds TextImage and GraphicsImage (subclasses of BaseImage) in .image.
  • Moves _pixel_ratio definitions into category baseclasses and turns BaseImage._pixel_ratio into an abstract property.
  • Changes the baseclass of TermImage to TextImage.
  • Changes the baseclass of KittyImage to GraphicsImage.
• Improves render style selection and support detection.
• Adds --force-style.
• Changes -S from --scroll to --style.
• Documents render style support.