enhance(astToPlainText): improved ReadMe block serialization

[rafegoldberg]

📖 PR App	🎫 RM-9982	:octocat: See ReadMe#12167

🧰 Changes

• [x] expose variable data off of the astToPlainText RDMD processor
• [x] improved plaintext serialization of custom ReadMe blocks
  • [x] variables
  • [x] glossary terms
  • [x] callouts

🧬 QA & Testing

Pull down this branch and try running the following snippet. Compare the results to what you see in next. If ReadMe variables show up as plain text in the output, this worked! :tada:

const body = `> 👍 Success
>
> [Vitae](http://www.google.com) *reprehenderit* at aliquid error voluptates eum dignissimos.

variable: <<name>>
glossary: <<glossary:myMadeUpTerm>>
`;

rdmd.astToPlainText(rdmd.hast(body, { strip: ["style"] }), {
  variables: { defaults: [], user: { name: "John Doe" } },
});

[rafegoldberg]

This PR was released!

🚀 Changes included in v6.86.0