deb822 Lexer Request

[tianon]

deb822

• https://manpages.debian.org/unstable/dpkg-dev/deb822.5.en.html
• (inspired by) https://www.ietf.org/rfc/rfc2822.txt
• https://manpages.debian.org/unstable/apt/sources.list.5.en.html#DEB822-STYLE_FORMAT
• https://manpages.debian.org/unstable/apt/apt_preferences.5.en.html
• https://salsa.debian.org/ci-team/autopkgtest/-/blob/a437a28172ad36d68b8dfcdfa233b19dd7239ba3/doc/README.package-tests.rst
• ... (many other tools inside Debian use this format, and some outside although they're less common)

Implementation in other libraries

• https://github.com/rouge-ruby/rouge/blob/2a8b21b530076f0df4515397482651099ce789b2/lib/rouge/lexers/email.rb 😇
• https://pygments.org/docs/lexers/#pygments.lexers.installers.DebianSourcesLexer

Additional context
I think this should be pretty easy to factor out of the existing email lexer, but perhaps the devil's in the details somewhere I can't see. 🙇

For now, I'm getting by with either "yaml" or "email" depending on whether I've got more than one "stanza" ❤️