LibWeb: Make CSS Transformation struct a proper class

Move it out of ComputedValues.h into its own files, and take the
transformation-to-matrix code from StackingContext.

Userland/Libraries/LibWeb/Forward.h

@@ -166,6 +166,7 @@ class Time;
 class TimeOrCalculated;
 class TimePercentage;
 class TimeStyleValue;
+class Transformation;
 class TransformationStyleValue;
 class URLStyleValue;
 class UnicodeRange;