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/CSSStyleSheet.cpp

@@ -112,6 +112,12 @@ void CSSStyleSheet::for_each_effective_style_rule(Function<void(CSSStyleRule con
     }
 }
 
+void CSSStyleSheet::for_each_effective_keyframes_at_rule(Function<void(CSSKeyframesRule const&)> const& callback) const
+{
+    if (m_media->matches())
+        m_rules->for_each_effective_keyframes_at_rule(callback);
+}
+
 bool CSSStyleSheet::evaluate_media_queries(HTML::Window const& window)
 {
     bool any_media_queries_changed_match_state = false;