LibWeb: Move PositionValue into its own files

It's in Position.{h,cpp} because it represents a <position> in CSS, even
though it's currently named PositionValue to avoid collisions.

Userland/Libraries/LibWeb/CSS/Position.h

@@ -0,0 +1,53 @@
+/*
+ * Copyright (c) 2022-2023, MacDue <macdue@dueutil.tech>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#pragma once
+
+#include <LibWeb/CSS/Percentage.h>
+#include <LibWeb/PixelUnits.h>
+
+namespace Web::CSS {
+
+// FIXME: Named PositionValue to avoid conflicts with enums, but this represents a <position>
+struct PositionValue {
+    enum class HorizontalPreset {
+        Left,
+        Center,
+        Right
+    };
+
+    enum class VerticalPreset {
+        Top,
+        Center,
+        Bottom
+    };
+
+    enum class HorizontalEdge {
+        Left,
+        Right
+    };
+
+    enum class VerticalEdge {
+        Top,
+        Bottom
+    };
+
+    static PositionValue center()
+    {
+        return PositionValue { HorizontalPreset::Center, VerticalPreset::Center };
+    }
+
+    Variant<HorizontalPreset, LengthPercentage> horizontal_position { HorizontalPreset::Left };
+    Variant<VerticalPreset, LengthPercentage> vertical_position { VerticalPreset::Top };
+    HorizontalEdge x_relative_to { HorizontalEdge::Left };
+    VerticalEdge y_relative_to { VerticalEdge::Top };
+
+    CSSPixelPoint resolved(Layout::Node const& node, CSSPixelRect const& rect) const;
+    ErrorOr<void> serialize(StringBuilder&) const;
+    bool operator==(PositionValue const&) const = default;
+};
+
+}