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.test.tsx

@@ -0,0 +1,51 @@
+import assert from "node:assert/strict";
+import { suite, test } from "node:test";
+import { resolveRefs } from "./resolve-refs.js";
+import type { Element } from "antihtml";
+
+
+suite("function resolveRefs", () => {
+	test("root to root relative href", () => {
+		const el = resolveRefs(<a href="/page.html">Link</a>, "/");
+		assert.equal((el as Element).attributes.get("href"), "page.html");
+	});
+
+	test("root to subdir relative href", () => {
+		const el = resolveRefs(<a href="/dir/page.html">Link</a>, "/");
+		assert.equal((el as Element).attributes.get("href"), "dir/page.html");
+	});
+
+	test("subdir to root relative href", () => {
+		const el = resolveRefs(<a href="/page.html">Link</a>, "/subdir");
+		assert.equal((el as Element).attributes.get("href"), "../page.html");
+	});
+
+	test("subdir to subdir relative href", () => {
+		const el = resolveRefs(<a href="/alt/page.html">Link</a>, "/subdir");
+		assert.equal((el as Element).attributes.get("href"), "../alt/page.html");
+	});
+
+	test("nested element", () => {
+		const el = resolveRefs(<div>Content with <a href="/page.html">Link</a></div>, "/");
+		assert.equal((el.childNodes[1] as Element).attributes.get("href"), "page.html");
+	});
+
+	test("returns element if no changes", () => {
+		const el = <div>Content with <em>emphasis</em></div>;
+		const resEl = resolveRefs(el, "/");
+		assert.equal(el, resEl);
+	});
+
+	test("returns new element if changed", () => {
+		const el = <div>Content with <a href="/page.html">Link</a></div>;
+		const resEl = resolveRefs(el, "/");
+		assert.notEqual(el, resEl);
+	});
+
+	test("does not modify input", () => {
+		const elFn = () => <div>Content with <a href="/page.html">Link</a></div>;
+		const el = elFn();
+		resolveRefs(el, "/");
+		assert.deepEqual(el, elFn());
+	});
+});