@actions/core: convert to ESM module

__tests__/summary.test.ts

@@ -1,7 +1,7 @@
 import * as fs from 'fs'
 import * as os from 'os'
 import path from 'path'
-import {summary, SUMMARY_ENV_VAR} from '../src/summary'
+import {summary, SUMMARY_ENV_VAR} from '../src/summary.js'
 
 const testDirectoryPath = path.join(__dirname, 'test')
 const testFilePath = path.join(testDirectoryPath, 'test-summary.md')