LibWeb: Introduce CalculatedOr type

This is intended as a replacement for Length and friends each holding a
`RefPtr<CalculatedStyleValue>`. Instead, let's make the types explicit
about whether they are calculated or not. This then means a Length is
always a Length, and won't require including `StyleValue.h`.

As noted, it's probably nicer for LengthOrCalculated to live in
`Length.h`, but we can't do that until Length stops including
`StyleValue.h`.

Userland/Libraries/LibWeb/Forward.h

@@ -28,6 +28,7 @@ namespace Web::CSS {
 class AbstractImageStyleValue;
 class Angle;
 class AnglePercentage;
+class AngleOrCalculated;
 class AngleStyleValue;
 class BackgroundRepeatStyleValue;
 class BackgroundSizeStyleValue;
@@ -61,6 +62,7 @@ class FontFace;
 class FontStyleValue;
 class Frequency;
 class FrequencyPercentage;
+class FrequencyOrCalculated;
 class FrequencyStyleValue;
 class GridAreaShorthandStyleValue;
 class GridMinMax;
@@ -79,6 +81,7 @@ class InitialStyleValue;
 class Length;
 class LengthBox;
 class LengthPercentage;
+class LengthOrCalculated;
 class LengthStyleValue;
 class LinearGradientStyleValue;
 class ListStyleStyleValue;
@@ -91,6 +94,7 @@ class Number;
 class NumericStyleValue;
 class OverflowStyleValue;
 class Percentage;
+class PercentageOrCalculated;
 class PercentageStyleValue;
 class PositionStyleValue;
 class PropertyOwningCSSStyleDeclaration;
@@ -113,6 +117,7 @@ class Supports;
 class TextDecorationStyleValue;
 class Time;
 class TimePercentage;
+class TimeOrCalculated;
 class TimeStyleValue;
 class TransformationStyleValue;
 class UnicodeRange;