Fabric/Text: attributedstring module, the first part

Summary:
`fabric/attributedstring` is a simple, cross-platfrom, react-specific implementation of attributed string (aka spanned string).
This diff is the first part of this which contains text primitives (types) and conversions.

Reviewed By: fkgozali

Differential Revision: D7748704

fbshipit-source-id: d76e31807e5ac7ab1a16fd6ee6445c59de5b89a2

ReactCommon/fabric/attributedstring/tests/AttributedStringTest.cpp

@@ -0,0 +1,18 @@
+/**
+ * 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.
+ */
+
+#include <memory>
+
+#include <gtest/gtest.h>
+
+#include <fabric/attributedstring/TextPrimitives.h>
+
+using namespace facebook::react;
+
+TEST(AttributedStringTest, testSomething) {
+  // TODO
+}