@actions/io: convert to ESM module

__tests__/io.test.ts

@@ -2,8 +2,8 @@ import * as child from 'child_process'
 import {promises as fs} from 'fs'
 import * as os from 'os'
 import * as path from 'path'
-import * as io from '../src/io'
-import * as ioUtil from '../src/io-util'
+import * as io from '../src/io.js'
+import * as ioUtil from '../src/io-util.js'
 
 describe('cp', () => {
   beforeAll(async () => {