feat(myst plugin): Implement working MySt Plugin

[vijay-prema]

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[nokome]

Thanks for this @vijay-prema.

A little bug I noticed: when encoding, there is no blank line after admonition, table, figure etc. These need to be encoded with a \n\n at the end like paragraph does.

There are a few Stencila node types for which encoding is not implemented but which can be. We should ensure these types are also handled in decoding also (i.e. ensure we have full roundtrip encoding/decoding).

• Block node types:

  • Claim (analogous to a MyST "proof" see https://mystmd.org/guide/proofs-and-theorems, note there are different claim types)
  • CodeChunk (equivalent to a MyST code-cell see https://mystmd.org/guide/notebooks-with-markdown)
  • IncludeBlock (equivalent to an embed directive see https://mystmd.org/guide/embed)
  • QuoteBlock (supported by MyST see https://mystmd.org/guide/typography#quotations)
  • Section (equivalent to MyST block see https://mystmd.org/guide/blocks)

• Inline node types:

  • AudioObject & VideoObject (encode like an image, this is what we do for Stencila Markdown also, see https://mystmd.org/guide/figures#videos)
  • Cite & CiteGroup (we don't have support in Stencila Markdown yet, but may as well implement encoding since MyST does https://mystmd.org/guide/citations#markdown-citations)
  • CodeExpression (equivalent to a MyST {eval} https://mystmd.org/guide/notebooks-with-markdown#myst-inline-expressions)
  • Note (this includes footnotes which MyST supports see https://mystmd.org/guide/typography#footnotes)