LibHTML: Implement the "margin" shorthand property

This is a very bulky way of doing this, and doesn't seem sustainable to
implement every shorthand property this way, but it's a place to start.

The "margin" CSS property now expands into its four longhands as far as
my understanding of the specs.

Note that shorthand expansion happens when we *resolve* style, not when
we parse CSS. I'm not sure this is correct anymore, I think other UA's
may actually expand shorthands into the declaration directly at parse
these days. If so, we should do this at parsing as well.

Libraries/LibHTML/CSS/StyleValue.h

@@ -46,6 +46,8 @@ public:
     bool is_color() const { return type() == Type::Color; }
     bool is_identifier() const { return type() == Type::Identifier; }
     bool is_image() const { return type() == Type::Image; }
+    bool is_string() const { return type() == Type::String; }
+    bool is_length() const { return type() == Type::Length; }
 
     virtual String to_string() const = 0;
     virtual Length to_length() const { return {}; }