LibWeb: Add KeyframeEffect IDL objects

Userland/Libraries/LibWeb/Animations/KeyframeEffect.idl

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+#import <Animations/AnimationEffect.idl>
+#import <DOM/Element.idl>
+
+// https://www.w3.org/TR/web-animations-1/#the-compositeoperation-enumeration
+enum CompositeOperation { "replace", "add", "accumulate" };
+
+// https://www.w3.org/TR/web-animations-1/#enumdef-compositeoperationorauto
+enum CompositeOperationOrAuto { "replace", "add", "accumulate", "auto" };
+
+// https://www.w3.org/TR/web-animations-1/#the-keyframeeffectoptions-dictionary
+dictionary KeyframeEffectOptions : EffectTiming {
+    CompositeOperation composite = "replace";
+    CSSOMString?       pseudoElement = null;
+};
+
+// https://www.w3.org/TR/web-animations-1/#dictdef-basepropertyindexedkeyframe
+dictionary BasePropertyIndexedKeyframe {
+    (double? or sequence<double?>)                         offset = [];
+    (DOMString or sequence<DOMString>)                     easing = [];
+    (CompositeOperationOrAuto or sequence<CompositeOperationOrAuto>) composite = [];
+};
+
+// https://www.w3.org/TR/web-animations-1/#dictdef-basekeyframe
+dictionary BaseKeyframe {
+    double?                  offset = null;
+    DOMString                easing = "linear";
+    CompositeOperationOrAuto composite = "auto";
+};
+
+// https://www.w3.org/TR/web-animations-1/#the-keyframeeffect-interface
+[Exposed=Window]
+interface KeyframeEffect : AnimationEffect {
+    constructor(Element? target,
+                object? keyframes,
+                optional (unrestricted double or KeyframeEffectOptions) options = {});
+    constructor(KeyframeEffect source);
+
+    attribute Element?           target;
+    attribute CSSOMString?       pseudoElement;
+    attribute CompositeOperation composite;
+    sequence<object> getKeyframes();
+    undefined        setKeyframes(object? keyframes);
+};