LibWeb/CSS: Add basic implementation of CSSMarginRule

This is a bit under-specced, specifically there's no definition of
CSSMarginDescriptors so I've gone with CSSStyleProperties for now. Gets
us 17 WPT subtests.

Libraries/LibWeb/Forward.h

@@ -200,6 +200,7 @@ class CSSKeyframesRule;
 class CSSKeywordValue;
 class CSSLayerBlockRule;
 class CSSLayerStatementRule;
+class CSSMarginRule;
 class CSSMediaRule;
 class CSSNamespaceRule;
 class CSSNestedDeclarations;