feat: format css

A port of bun's port of Prettier's CSS printer and the four parsers it
uses, as tidyrs::css with Css, Scss and Less dialects. Statements are
written straight to the output when every group on a line fits flat,
as bun does. One that doesn't goes through a small builder into the
shared doc IR, which print_at prints in place.

Markup now formats <style> and style="" with it unless the embed hook
answers first, and embeds the CSS document rather than its text.
format_embedded is Prettier's embeddedLanguageFormatting: off, neither
the hook nor the built-in formatters run. All formatters keep a byte
order mark, as Prettier does.

Conformance: CSS 158/160, SCSS 135/136, Less 236/238. The misses are
bun's corrections of custom property values, and the same embedded in
markup matches Prettier fixture for fixture.