dispatch a load event when href set for Link element

Also add `lazy-href-set` test.

src/browser/tests/element/html/link.html

@@ -69,3 +69,18 @@
   testing.eventually(() => testing.expectEqual(false, fired));
 }
 </script>
+
+<script id="lazy-href-set">
+{
+  let result = false;
+  const link = document.createElement("link");
+  link.rel = "stylesheet";
+  link.onload = () => result = true;
+  // Append to DOM,
+  document.head.appendChild(link);
+  // then set href.
+  link.href = 'https://lightpanda.io/opensource-browser/15';
+
+  testing.eventually(() => testing.expectEqual(true, result));
+}
+</script>

src/browser/webapi/element/html/Link.zig

@@ -50,7 +50,12 @@ pub fn getHref(self: *Link, page: *Page) ![]const u8 {
 }
 
 pub fn setHref(self: *Link, value: []const u8, page: *Page) !void {
-    try self.asElement().setAttributeSafe(comptime .wrap("href"), .wrap(value), page);
+    const element = self.asElement();
+    try element.setAttributeSafe(comptime .wrap("href"), .wrap(value), page);
+
+    if (element.asNode().isConnected()) {
+        try page.linkAddedCallback(self);
+    }
 }
 
 pub fn getRel(self: *Link) []const u8 {