fix: include index.js in files array

This repairs the ability to `node .` from the installed package.

`require('npm')` will still not work, as this was disabled in v8.0.0

PR-URL: #3874
Credit: @wraithgar
Close: #3874
Reviewed-by: @lukekarrys

Author: wraithgar

package.json

@@ -7,6 +7,7 @@
     "packages/*"
   ],
   "files": [
+    "index.js",
     "bin",
     "docs/content/**/*.md",
     "docs/output/**/*.html",