Add types property to use generated declaration file rather than source (#6)

See #6 (comment)

Author: Levertion

package-lock.json

@@ -2,7 +2,8 @@
   "name": "node-interval-tree",
   "version": "1.3.2",
   "description": "Implementation of interval tree data structure.",
-  "main": "lib/index.js",
+  "main": "./lib/index.js",
+  "types": "./lib/index.d.ts",
   "engines": {
     "node": ">= 7.6.0"
   },