LibWeb: Add preliminary support for CSS animations

This partially implements CSS-Animations-1 (though there are references
to CSS-Animations-2).
Current limitations:
- Multi-selector keyframes are not supported.
- Most animation properties are ignored.
- Timing functions are not applied.
- Non-absolute values are not interpolated unless the target is also of
  the same non-absolute type (e.g. 10% -> 25%, but not 10% -> 20px).
- The JavaScript interface is left as an exercise for the next poor soul
  looking at this code.

With those said, this commit implements:
- Interpolation for most common types
- Proper keyframe resolution (including the synthetic from-keyframe
  containing the initial state)
- Properly driven animations, and proper style invalidation

Co-Authored-By: Andreas Kling <kling@serenityos.org>

Userland/Libraries/LibWeb/CSS/CSSKeyframesRule.cpp

@@ -0,0 +1,36 @@
+/*
+ * Copyright (c) 2023, Ali Mohammad Pur <mpfard@serenityos.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#include "CSSKeyframesRule.h"
+
+namespace Web::CSS {
+
+void CSSKeyframesRule::visit_edges(Visitor& visitor)
+{
+    Base::visit_edges(visitor);
+    for (auto& keyframe : m_keyframes)
+        visitor.visit(keyframe);
+}
+
+JS::ThrowCompletionOr<void> CSSKeyframesRule::initialize(JS::Realm&)
+{
+    return {};
+}
+
+DeprecatedString CSSKeyframesRule::serialized() const
+{
+    StringBuilder builder;
+    builder.appendff("@keyframes \"{}\"", name());
+    builder.append(" { "sv);
+    for (auto& keyframe : keyframes()) {
+        builder.append(keyframe->css_text());
+        builder.append(' ');
+    }
+    builder.append('}');
+    return builder.to_deprecated_string();
+}
+
+}