Add spacing around selector combinators

[bartveneman]

closes #7

• Formats selectors like a + b and :where(a + b)
• Formats selectors with Nth, like li:nth-child(2n of .noted) and p:nth-child(1n + 8):nth-child(-1n + 15)
• Fixes issue where Raw rule prelude wouldn't be rendered at all, like li:nth-child() {} (was rendered as {} before this)