Add index.d.ts as "types" field of package.json.

Author: Jackson Kearl

package.json

@@ -6,6 +6,7 @@
   "private": true,
   "main": "index",
   "module": "index.mjs",
+  "types": "index.d.ts",
   "sideEffects": false,
   "homepage": "https://github.com/graphql/graphql-js",
   "bugs": {