Specify what to include in the published package with files attribute

Author: ryu1kn

.npmignore

@@ -26,6 +26,9 @@
     "url": "https://github.com/ryu1kn/multiline-string/issues"
   },
   "homepage": "https://github.com/ryu1kn/multiline-string#readme",
+  "files": [
+    "index.js"
+  ],
   "devDependencies": {
     "coveralls": "^3.0.11",
     "eslint": "^6.8.0",