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>

Base/res/html/misc/welcome.html

@@ -163,6 +163,7 @@
             <li><a href="inline-node.html">Styling "inline" elements</a></li>
             <li><a href="pseudo-elements.html">Pseudo-elements (::before, ::after, etc)</a></li>
             <li><a href="effects_with_opacity_and_transforms.html">Effects with opacity and transforms</a></li>
+            <li><a href="css-animations.html">CSS Animations</a></li>
         </ul>
 
         <h2>JavaScript/Wasm</h2>