RADIOACTIVE: Plan to remove SPCSSAttr and replace with SPStyle

See code changes for details on what this project entails.

The end goal is the complete removal of SPCSSAttr and replacement with a more capable SPStyle.

This will involve writing so many tests, you'll get bored scrolling through them in the code review. ☺️ But planning to expand the SPStyle test suite will be important too.

People who need to be involved in reviewing this BEFORE a lot of code is written: @pbs3141 @S-Rafael @Tavmjong