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/Parser/TokenStream.h

@@ -54,11 +54,18 @@ public:
         bool m_commit { false };
     };
 
-    explicit TokenStream(Vector<T> const& tokens)
+    explicit TokenStream(Span<T const> tokens)
         : m_tokens(tokens)
         , m_eof(make_eof())
     {
     }
+
+    explicit TokenStream(Vector<T> const& tokens)
+        : m_tokens(tokens.span())
+        , m_eof(make_eof())
+    {
+    }
+
     TokenStream(TokenStream<T> const&) = delete;
     TokenStream(TokenStream<T>&&) = default;
 
@@ -128,7 +135,7 @@ public:
     }
 
 private:
-    Vector<T> const& m_tokens;
+    Span<T const> m_tokens;
     int m_iterator_offset { -1 };
 
     T make_eof()