[Markdoc] `headings` and heading IDs (#7095)

* deps: markdown-remark

* wip: heading-ids function

* chore: add `@astrojs/markdoc` to external

* feat: `headings` support

* fix: allow `render` config on headings

* fix: nonexistent `userConfig`

* test: headings, toc, astro component render

* docs: README

* chore: changeset

* refactor: expose Markdoc helpers from runtime

* fix: bad named exports (commonjsssss)

* refactor: defaultNodes -> nodes

* deps: github-slugger

* fix: reset slugger cache on each render

* fix: bad astroNodes import

* docs: explain headingSlugger export

* docs: add back double stringify comment

* chore: bump to minor for internal exports change

1 files changed, 42 insertions, 0 deletions

diff --git a/packages/integrations/markdoc/src/nodes/heading.ts b/packages/integrations/markdoc/src/nodes/heading.ts
new file mode 100644
index 000000000..81a9181c7
--- /dev/null
+++ b/packages/integrations/markdoc/src/nodes/heading.ts
@@ -0,0 +1,42 @@
+import Markdoc, { type RenderableTreeNode, type Schema } from '@markdoc/markdoc';
+import { getTextContent } from '../runtime.js';
+import Slugger from 'github-slugger';
+
+export const headingSlugger = new Slugger(); 
+
+function getSlug(attributes: Record<string, any>, children: RenderableTreeNode[]): string {
+	if (attributes.id && typeof attributes.id === 'string') {
+		return attributes.id;
+	}
+	const textContent = attributes.content ?? getTextContent(children);
+	let slug = headingSlugger.slug(textContent);
+
+	if (slug.endsWith('-')) slug = slug.slice(0, -1);
+	return slug;
+}
+
+export const heading: Schema = {
+	children: ['inline'],
+	attributes: {
+		id: { type: String },
+		level: { type: Number, required: true, default: 1 },
+	},
+	transform(node, config) {
+		const { level, ...attributes } = node.transformAttributes(config);
+		const children = node.transformChildren(config);
+
+
+		const slug = getSlug(attributes, children);
+
+		const render = config.nodes?.heading?.render ?? `h${level}`;
+		const tagProps =
+			// For components, pass down `level` as a prop,
+			// alongside `__collectHeading` for our `headings` collector.
+			// Avoid accidentally rendering `level` as an HTML attribute otherwise!
+			typeof render === 'function'
+				? { ...attributes, id: slug, __collectHeading: true, level }
+				: { ...attributes, id: slug };
+
+		return new Markdoc.Tag(render, tagProps, children);
+	},
+};