LibWeb: Compute the final border-style property before painting

Instead of doing a CSS property lookup for the line style of each
border edge during paint, we now cache the final CSS::LineStyle to use
in the Layout::BorderData.

Libraries/LibWeb/CSS/StyleValue.h

@@ -149,6 +149,19 @@ enum class Float {
     Right,
 };
 
+enum class LineStyle {
+    None,
+    Hidden,
+    Dotted,
+    Dashed,
+    Solid,
+    Double,
+    Groove,
+    Ridge,
+    Inset,
+    Outset,
+};
+
 class StyleValue : public RefCounted<StyleValue> {
 public:
     virtual ~StyleValue();