LibWeb: Add EdgeStyleValue

This represents a single edge and offset, this will be needed for
the values of background-position-x/y.

Userland/Libraries/LibWeb/CSS/StyleValues/EdgeStyleValue.cpp

@@ -0,0 +1,30 @@
+/*
+ * Copyright (c) 2023, MacDue <macdue@dueutil.tech>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#include "EdgeStyleValue.h"
+
+namespace Web::CSS {
+
+ErrorOr<String> EdgeStyleValue::to_string() const
+{
+    auto to_string = [](PositionEdge edge) {
+        switch (edge) {
+        case PositionEdge::Left:
+            return "left";
+        case PositionEdge::Right:
+            return "right";
+        case PositionEdge::Top:
+            return "top";
+        case PositionEdge::Bottom:
+            return "bottom";
+        }
+        VERIFY_NOT_REACHED();
+    };
+
+    return String::formatted("{} {}", to_string(m_properties.edge), TRY(m_properties.offset.to_string()));
+}
+
+}