feat! parse Markdown input with pull-cmark instead of Pandoc
Description
For Sequoia, depending on Pandoc to parse Markdown is a problem, as it's quite a large build dependency. This replaces Pandoc with the pulldown-cmark crate, for codegen only. If this works out in practice, I'm going to extend the parser for docgen as well so we only use one parser for Markdown.
Checklist
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You have read, and followed the guidance in, the CONTRIBUTING guide -
The MR content has run through a pipeline, or at least has pipelines enabled. -
There are no licence concerns with this contribution (The README clearly states Subplot is GPL-3.0)