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/Enums.json

@@ -32,6 +32,18 @@
         "stretch",
         "unsafe"
     ],
+    "animation-fill-mode": [
+        "backwards",
+        "both",
+        "forwards",
+        "none"
+    ],
+    "animation-direction": [
+        "alternate",
+        "alternate-reverse",
+        "normal",
+        "reverse"
+    ],
     "appearance": [
         "auto",
         "button",