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/Identifiers.json

@@ -61,9 +61,12 @@
   "alias",
   "all",
   "all-scroll",
+  "alternate",
+  "alternate-reverse",
   "anywhere",
   "auto",
   "back",
+  "backwards",
   "baseline",
   "blink",
   "block",
@@ -109,6 +112,10 @@
   "dotted",
   "double",
   "e-resize",
+  "ease",
+  "ease-in",
+  "ease-in-out",
+  "ease-out",
   "enabled",
   "end",
   "ew-resize",
@@ -126,6 +133,7 @@
   "flow",
   "flow-root",
   "from-font",
+  "forwards",
   "full-size-kana",
   "full-width",
   "fullscreen",
@@ -161,6 +169,7 @@
   "less",
   "light",
   "lighter",
+  "linear",
   "line-through",
   "list-item",
   "local",
@@ -204,6 +213,7 @@
   "p3",
   "padding-box",
   "paged",
+  "paused",
   "pixelated",
   "pointer",
   "portrait",
@@ -220,6 +230,7 @@
   "repeat",
   "repeat-x",
   "repeat-y",
+  "reverse",
   "ridge",
   "right",
   "round",
@@ -232,6 +243,7 @@
   "ruby-base-container",
   "ruby-text",
   "ruby-text-container",
+  "running",
   "run-in",
   "radio",
   "s-resize",