Add Platform module for web target (#23387)

Summary:
Added a Platform file for module for instances where internal modules need Platform.
ex: https://github.com/facebook/react-native/blob/78be6efda97096e8388af5bf2dadb9c9de4fa0ba/Libraries/Utilities/Dimensions.js#L14

* Added `Libraries/Utilities/Platform.web.js`
[CATEGORY] [TYPE] - Message
Pull Request resolved: #23387

Differential Revision: D14041834

Pulled By: cpojer

fbshipit-source-id: 9be7ad647d6a644956efa622b936fcecee7c1a1f

Author: EvanBacon

Libraries/Utilities/Platform.web.js

@@ -0,0 +1,24 @@
+/**
+ * Copyright (c) 2015-present, Facebook, Inc.
+ *
+ * This source code is licensed under the MIT license found in the
+ * LICENSE file in the root directory of this source tree.
+ *
+ * @format
+ * @flow
+ */
+
+'use strict';
+
+export type PlatformSelectSpec<D, I> = {
+  default?: D,
+  web?: I,
+};
+
+const Platform = {
+  OS: 'web',
+  select: <D, I>(spec: PlatformSelectSpec<D, I>): D | I =>
+    'web' in spec ? spec.web : spec.default,
+};
+
+module.exports = Platform;