@actions/core: convert to ESM module

__tests__/core.test.ts

@@ -1,9 +1,9 @@
 import * as fs from 'fs'
 import * as os from 'os'
 import * as path from 'path'
-import * as core from '../src/core'
+import * as core from '../src/core.js'
 import {HttpClient} from '@actions/http-client'
-import {toCommandProperties} from '../src/utils'
+import {toCommandProperties} from '../src/utils.js'
 
 /* eslint-disable @typescript-eslint/unbound-method */