Add resolveRefs transform

Provide a transformation function that maps absolute references to
resources into relative references based on the location of a page.
This makes it possible to use the same links across multiple pages
in the hierarchy that works when loaded as files from the filesystem.

utils/resolve-refs.ts

@@ -0,0 +1,52 @@
+import * as posix from "node:path/posix";
+import { Node, Element } from "antihtml";
+
+function shallowCopyElement(element: Element) {
+	const copy = new Element(element.name);
+	copy.attributes = new Map(element.attributes);
+	copy.childNodes = element.childNodes;
+	return copy;
+}
+
+/**
+	Resolves absolute href attributes in a and link elements the Node tree into relative references from the given directory.
+
+	@param node Node tree to transform
+	@param dir Absolute path to directory to resolve references from.
+	@returns new node tree with href attributes transformed, or the original node if no transformations took place.
+*/
+export function resolveRefs(node: Node, dir: string) {
+	if (!(node instanceof Element)) {
+		return node;
+	}
+	let resolvedNode = node;
+	const name = node.name;
+	if (
+		(name === "link" || name === "a")
+		&& node.attributes.has("href")
+	) {
+		const original = node.attributes.get("href")!
+		/* node:coverage ignore next 3 */
+		if (!original.startsWith("/")) {
+			console.log(`Warning: found relative href to ${original}`);
+		} else {
+			const ref = posix.relative(dir, original);
+			resolvedNode = shallowCopyElement(node);
+			resolvedNode.attributes.set("href", ref);
+		}
+	}
+	const resolvedChildren: Node[] = [];
+	let modifiedChildren = false;
+	for (const child of resolvedNode.childNodes) {
+		const resolvedChild = resolveRefs(child, dir);
+		if (child !== resolvedChild) {
+			modifiedChildren = true;
+		}
+		resolvedChildren.push(resolvedChild);
+	}
+	if (modifiedChildren) {
+		resolvedNode = shallowCopyElement(node);
+		resolvedNode.childNodes = resolvedChildren;
+	}
+	return resolvedNode;
+}