LibWeb: Implement initial CSSFontFaceRule and FontFace classes

For now, this is the bare minimum that's needed: font-family and src.

Userland/Libraries/LibWeb/Forward.h

@@ -31,6 +31,7 @@ class CalculatedStyleValue;
 class ColorStyleValue;
 class ContentStyleValue;
 class CSSImportRule;
+class CSSFontFaceRule;
 class CSSMediaRule;
 class CSSRule;
 class CSSRuleList;
@@ -42,6 +43,7 @@ class Display;
 class ElementInlineCSSStyleDeclaration;
 class FlexFlowStyleValue;
 class FlexStyleValue;
+class FontFace;
 class FontStyleValue;
 class Frequency;
 class FrequencyPercentage;
@@ -381,6 +383,7 @@ class CharacterDataWrapper;
 class CloseEventWrapper;
 class CommentWrapper;
 class CryptoWrapper;
+class CSSFontFaceRuleWrapper;
 class CSSRuleListWrapper;
 class CSSRuleWrapper;
 class CSSStyleDeclarationWrapper;