Generate in-code function descriptions from the Markdown documentation

[aleclarson]

Instead of maintaining both inline documentation and Markdown documentation for a function, I'd like to generate one from the other. I think the Markdown-to-inline direction makes most sense.

Key points

• The entire documentation for a function should be copied over at build-time.
• We'll probably need an ESBuild plugin with Markdown AST and TypeScript AST parsing involved. I'm also a fan of magic-string for code editing, since it can generate a sourcemap when we need it (which is whenever we generate sourcemaps ourselves).
• We need special handling for some things, like Astro callouts.
• It's probably fine if we truncate the documentation at some point, then link to the website. Maybe use an explicit HTML comment in the Markdown files (like <!-- cut here -->), so we can control the cut-off point.