Separate source files into directories.
Update description and tags (Mention 'preprocessing.')
Update README (Better JSCSS introduction.)
fix: Make test-3, property-1a pass.
Make test-3 properties 2a, 3a, and 3c pass.
Improve test-3 properties 2b, 3b, and 3d support.
Make test-3 properties 2b, 3b, and 3d pass.
refactor: Remove redundant CSSPropertyValueBalancedPairPart.