chore: add files and keywords to package.json

Author: wemeetagain

package.json

@@ -3,6 +3,16 @@
   "version": "2.0.0",
   "main": "index.js",
   "types": "index.d.ts",
+  "files": [
+    "index.js",
+    "index.d.ts"
+  ],
+  "keywords": [
+    "ethereum",
+    "lodestar",
+    "napi",
+    "N-API"
+  ],
   "napi": {
     "name": "pubkey-index-map",
     "triples": {