chore: assert index.ts is running in GitHub Action

Author: tony19

src/index.ts

@@ -1,7 +1,10 @@
+import { assert } from './utils'
 import { createConfig } from './config'
 import { RyuCho } from './ryu-cho'
 import core from '@actions/core'
 
+assert(typeof core !== 'undefined', `core is undefined, which probably means you're not running in a GitHub Action`)
+
 const config = createConfig({
   accessToken: core.getInput('access-token', { required: true }),
   userName: core.getInput('username', { required: true }),