feat: wire YAML in

Adds Language::Yaml, a yaml config section with wrap, the yaml
parser to the CLI and binding, and YAML to the builtin embed, so
YAML front matter and yaml code blocks are formatted when the hook
declines, as Prettier formats them. Wrap moves to the crate root,
since Markdown and YAML share it, and bracketSpacing becomes a
shared option, since JSON and YAML follow it, as in Prettier.