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.idl

@@ -0,0 +1,13 @@
+#import <CSS/CSSRule.idl>
+
+[Exposed=Window]
+interface CSSKeyframesRule : CSSRule {
+    attribute CSSOMString name;
+    readonly attribute CSSRuleList cssRules;
+    readonly attribute unsigned long length;
+
+    getter CSSKeyframeRule (unsigned long index);
+    undefined        appendRule(CSSOMString rule);
+    undefined        deleteRule(CSSOMString select);
+    CSSKeyframeRule? findRule(CSSOMString select);
+};