CSS formatter

This is a structural formatter, not a linter or an optimizer. It will not fix invalid CSS, add missing semicolons to a broken rule, autoprefix, reorder properties, or drop comments; the minify mode in particular keeps comments on purpose, because deleted comments do not come back. Its whitespace dropping is deliberately conservative: spaces survive where removing them could change what a selector matches, such as the descendant space in `div :hover`. Unbalanced braces and unterminated strings or comments are reported with the line they start on, not guessed around.