LibWeb/CSS: Parse <url> as a new CSS::URL type

Our previous approach to `<url>` had a couple of issues:
- We'd complete the URL during parsing, when we should actually keep it
  as the original string until it's used.
- There's nowhere for us to store `<url-modifier>`s on a `URL::URL`.

So, `CSS::URL` is a solution to this. It holds the original URL string,
and later will also hold any modifiers. This commit parses all `<url>`s
as `CSS::URL`, but then converts it into a `URL::URL`, so no user code
is changed. These will be modified in subsequent commits.

For `@namespace`, we were never supposed to complete the URL at all, so
this makes that more correct already. However, in practice all
`@namespace`s are absolute URLs already, so this should have no
observable effects.

Libraries/LibWeb/CSS/URL.cpp

@@ -0,0 +1,31 @@
+/*
+ * Copyright (c) 2025, Sam Atkins <sam@ladybird.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#include <LibWeb/CSS/Serialize.h>
+#include <LibWeb/CSS/URL.h>
+
+namespace Web::CSS {
+
+URL::URL(String url)
+    : m_url(move(url))
+{
+}
+
+// https://drafts.csswg.org/cssom-1/#serialize-a-url
+String URL::to_string() const
+{
+    // To serialize a URL means to create a string represented by "url(", followed by the serialization of the URL as a string, followed by ")".
+    StringBuilder builder;
+    builder.append("url("sv);
+    serialize_a_string(builder, m_url);
+    builder.append(')');
+
+    return builder.to_string_without_validation();
+}
+
+bool URL::operator==(URL const&) const = default;
+
+}