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/css-animations.html

@@ -0,0 +1,50 @@
+<style>
+.system {
+  position: absolute;
+  width: 100%;
+  height: 100%;
+  background: #000;
+  overflow: hidden;
+}
+.buggie {
+  position: absolute;
+  width: 50%;
+  height: 50%;
+  scale: 50%;
+  opacity: 0;
+  background: url(https://serenityos.org/buggie.png) no-repeat left center;
+  background-size: contain;
+  animation: buggie 10s linear infinite;
+}
+.offset-0 { animation-delay: 0.9s; }
+.offset-1 { animation-delay: 1.7s; }
+.offset-2 { animation-delay: 3.5s; }
+.offset-3 { animation-delay: 4.3s; }
+
+.ladyball {
+  position: absolute;
+  width: 50%;
+  height: 50%;
+  background: url(https://upload.wikimedia.org/wikipedia/commons/thumb/b/b8/LadyBall-SerenityOS.png/240px-LadyBall-SerenityOS.png) no-repeat left center;
+  scale: 50%;
+  animation: ladyball 9s linear infinite;
+}
+@keyframes buggie {
+  0% { transform: translateX(0vw); opacity: 1; }
+  50% { transform: translateX(100vw); opacity: 1; }
+  100% { transform: translateX(0vw); opacity: 1; }
+}
+@keyframes ladyball {
+  0% { transform: translateX(0vw); }
+  50% { transform: translateX(100vw); }
+  100% { transform: translateX(0vw); }
+}
+</style>
+
+<div class=system>
+  <div class="buggie offset-0"></div>
+  <div class="buggie offset-1"></div>
+  <div class="buggie offset-2"></div>
+  <div class="buggie offset-3"></div>
+  <div class="ladyball"></div>
+</div>