feat: add types to package.json (cloudevents#216)

Signed-off-by: Grant Timmerman <timmerman+devrel@google.com>

Author: grant

Diff for: package.json

@@ -123,5 +123,6 @@
   },
   "publishConfig": {
     "access": "public"
-  }
+  },
+  "types": "index.d.ts"
 }