add: "types" to each package.json (#221)

fix es #148

package.json

@@ -10,6 +10,7 @@
   "homepage": "https://github.com/actions/toolkit/tree/master/packages/io",
   "license": "MIT",
   "main": "lib/io.js",
+  "types": "lib/io.d.ts",
   "directories": {
     "lib": "lib",
     "test": "__tests__"