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/CMakeLists.txt

@@ -22,6 +22,8 @@ set(SOURCES
     CSS/CSSConditionRule.cpp
     CSS/CSSGroupingRule.cpp
     CSS/CSSImportRule.cpp
+    CSS/CSSKeyframeRule.cpp
+    CSS/CSSKeyframesRule.cpp
     CSS/CSSFontFaceRule.cpp
     CSS/CSSMediaRule.cpp
     CSS/CSSRule.cpp