LibHTML: Add Length and LengthBox classes.

We need a way to represent values that are "auto", so adding a Length class
seems like the easiest way to achieve that.

LibHTML/CSS/Length.h

@@ -0,0 +1,26 @@
+#pragma once
+
+class Length {
+public:
+    enum class Type {
+        Auto,
+        Absolute,
+    };
+
+    Length() {}
+    Length(int value, Type type)
+        : m_type(type)
+        , m_value(value)
+    {
+    }
+    ~Length() {}
+
+    bool is_auto() const { return m_type == Type::Auto; }
+    bool is_absolute() const { return m_type == Type::Absolute; }
+
+    int value() const { return m_value; }
+
+private:
+    Type m_type { Type::Auto };
+    int m_value { 0 };
+};