Add copy link and highlight to comments

Opening a comment link expands the revision the comment is in, even if
the link is opened while already on the page, and scrolls to it.

This also fixes scrolling to URL fragments that aren't valid CSS
selectors, such as comment ids starting with a digit.

tests/unit/utils.test.ts

@@ -429,3 +429,22 @@ describe("untwemoji", () => {
     expect(node.innerHTML).toEqual("ab");
   });
 });
+
+describe("fragmentId", () => {
+  test.each([
+    { hash: "", expected: undefined },
+    { hash: "#", expected: undefined },
+    { hash: "#delegate", expected: "delegate" },
+    { hash: "delegate", expected: "delegate" },
+    {
+      hash: "#0a1b2c3d4e5f60718293a4b5c6d7e8f901234567",
+      expected: "0a1b2c3d4e5f60718293a4b5c6d7e8f901234567",
+    },
+    { hash: "#caf%C3%A9", expected: "café" },
+    { hash: "#%", expected: undefined },
+    { hash: "#%zz", expected: undefined },
+    { hash: "#%E0%A4%A", expected: undefined },
+  ])("fragmentId($hash) => $expected", ({ hash, expected }) => {
+    expect(utils.fragmentId(hash)).toEqual(expected);
+  });
+});