LibWeb: Implement initial CSSFontFaceRule and FontFace classes

For now, this is the bare minimum that's needed: font-family and src.

Userland/Libraries/LibWeb/CSS/CSSFontFaceRule.cpp

@@ -0,0 +1,29 @@
+/*
+ * Copyright (c) 2022, Sam Atkins <atkinssj@serenityos.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#include <LibWeb/CSS/CSSFontFaceRule.h>
+
+namespace Web::CSS {
+
+CSSFontFaceRule::CSSFontFaceRule(FontFace&& font_face)
+    : m_font_face(move(font_face))
+{
+}
+
+CSSStyleDeclaration* CSSFontFaceRule::style()
+{
+    // FIXME: Return a CSSStyleDeclaration subclass that directs changes to the FontFace.
+    return nullptr;
+}
+
+// https://www.w3.org/TR/cssom/#ref-for-cssfontfacerule
+String CSSFontFaceRule::serialized() const
+{
+    // FIXME: Implement this!
+    return "@font-face { /* FIXME: Implement CSSFontFaceRule::serialized()! */ }";
+}
+
+}