Migrate from @nuxtjs/mdc to Comark

@nuxtjs/mdc was Nuxt-only. Comark is its successor: the markdown syntax is fully compatible, your .md files need no changes. What changes is the JavaScript API.

The migration has two parts: Core Package (programmatic API) and Nuxt Module (components, slots, config).

Quick Overview

• Package: @nuxtjs/mdc → comark (core only) or @comark/nuxt (Nuxt module)
• Parse: parseMarkdown() → parseMarkdown() · factory: createMarkdownParser() (sync, no await)
• Render: stringifyMarkdown() → renderMarkdown() from comark/render
• AST: object tree → compact tuples ['tag', props, ...children]
• Result: result.body / result.data → document.nodes / document.frontmatter
• Renderer: <MDCRenderer :body :data> → <MarkdownDocument :value>
• All-in-one: <MDC :value> → <Markdown :value>
• Slots: <MDCSlot /> → native <slot />
• Plugins: global nuxt.config → per-component defineMarkdownComponent({ plugins })
• Markdown files: no changes needed