Markdown support for `AttributedString`

[parkera]

Factor out the Markdown support in AttributedString (which relies on a customized version of libcmark) and bring it to the package.

[QuietMisdreavus]

For what it's worth, the Markdown implementation of the AttributedString syntax is also open-source, and lives in the same Markdown library used by Swift-DocC: https://github.com/apple/swift-cmark