Add types property to indicate the main declaration file

Add `types` property to indicate the main declaration file.
[docs](http://www.typescriptlang.org/docs/handbook/declaration-files/publishing.html)

package.json

@@ -6,6 +6,7 @@
   "repository": "mrdoob/three.js",
   "jsnext:main": "build/three.module.js",
   "module": "build/three.module.js",
+  "types": "src/Three.d.ts",
   "files": [
     "package.json",
     "LICENSE",