LibHTML: Implement enough of the CSS parser to parse the default stylesheet.

LibHTML/CSS/StyleValue.h

@@ -1,9 +1,14 @@
 #pragma once
 
+#include <AK/AKString.h>
 #include <AK/RefCounted.h>
+#include <AK/RefPtr.h>
+#include <AK/StringView.h>
 
 class StyleValue : public RefCounted<StyleValue> {
 public:
+    static NonnullRefPtr<StyleValue> parse(const StringView&);
+
     virtual ~StyleValue();
 
     enum Type {
@@ -21,3 +26,18 @@ protected:
 private:
     Type m_type { Type::Invalid };
 };
+
+class PrimitiveStyleValue : public StyleValue {
+public:
+    virtual ~PrimitiveStyleValue() override {}
+    PrimitiveStyleValue(const String& string)
+        : StyleValue(Type::Primitive)
+        , m_string(string)
+    {
+    }
+
+    String to_string() const { return m_string; }
+
+private:
+    String m_string;
+};