Introduce CSSNode.Create API

Summary:
Add a convenience construction method for CSSNode.

    CSSNode node = CSSNode.Create(
        positionType: CSSPositionType.Absolute,
        wrap: CSSWrap.Wrap,
        position: new Spacing(top:6, right:4),
        margin: new Spacing(bottom:5, left:3));

Reviewed By: emilsjolander

Differential Revision: D4193380

fbshipit-source-id: 30b917f64e92997355a76e3b11799883b86fb9de

csharp/Facebook.CSSLayout/Spacing.cs

@@ -0,0 +1,31 @@
+/**
+ * Copyright (c) 2014-present, Facebook, Inc.
+ * All rights reserved.
+ *
+ * This source code is licensed under the BSD-style license found in the
+ * LICENSE file in the root directory of this source tree. An additional grant
+ * of patent rights can be found in the PATENTS file in the same directory.
+ */
+
+namespace Facebook.CSSLayout
+{
+    public class Spacing
+    {
+        public float? Top;
+        public float? Bottom;
+        public float? Left;
+        public float? Right;
+
+        public Spacing(
+            float? top = null,
+            float? bottom = null,
+            float? left = null,
+            float? right = null)
+        {
+            Top = top;
+            Bottom = bottom;
+            Left = left;
+            Right = right;
+        }
+    }
+}