Add TypeScript declarations

javascript/sources/index.d.ts

@@ -0,0 +1,15 @@
+/**
+ * Copyright (c) Meta Platforms, Inc. and affiliates.
+ *
+ * This source code is licensed under the MIT license found in the
+ * LICENSE file in the root directory of this source tree.
+ *
+ * @format
+ */
+
+import type {Yoga} from './wrapAsm';
+
+export * from './YGEnums';
+export * from './wrapAsm';
+
+export function load(): Promise<Yoga>;