LibWeb: Clarify StyleValue API with new naming scheme

This does a few things, that are hard to separate. For a while now, it's
been confuzing what `StyleValue::is_foo()` actually means. It sometimes
was used to check the type, and sometimes to see if it could return a
certain value type. The new naming scheme is:

- `is_length()` - is it a LengthStyleValue?
- `as_length()` - casts it to LengthStyleValue
- `has_length()` - can it return a Length?
- `to_length()` - gets the internal value out (eg, Length)

This also means, no more `static_cast<LengthStyleValue const&>(*this)`
stuff when dealing with StyleValues. :^)

Hopefully this will be a bit clearer going forward. There are lots of
places using the original methods, so I'll be going through them to
hopefully catch any issues.

Userland/Libraries/LibWeb/Forward.h

@@ -28,8 +28,33 @@ class Selector;
 class StyleProperties;
 class StyleResolver;
 class StyleSheet;
-class StyleValue;
 enum class Display;
+
+class StyleValue;
+class BackgroundRepeatStyleValue;
+class BackgroundStyleValue;
+class BorderRadiusStyleValue;
+class BorderStyleValue;
+class BoxShadowStyleValue;
+class CalculatedStyleValue;
+class ColorStyleValue;
+class CustomStyleValue;
+class FlexFlowStyleValue;
+class FlexStyleValue;
+class FontStyleValue;
+class IdentifierStyleValue;
+class ImageStyleValue;
+class InheritStyleValue;
+class InitialStyleValue;
+class LengthStyleValue;
+class ListStyleStyleValue;
+class NumericStyleValue;
+class OverflowStyleValue;
+class StringStyleValue;
+class TextDecorationStyleValue;
+class TransformationStyleValue;
+class UnsetStyleValue;
+class StyleValueList;
 }
 
 namespace Web::DOM {