Convert to ESM.

Context: https://github.com/actions/typescript-action/pull/969

__fixtures__/core.ts

@@ -0,0 +1,13 @@
+import type * as core from '@actions/core'
+import { jest } from '@jest/globals'
+
+export const debug = jest.fn<typeof core.debug>()
+export const error = jest.fn<typeof core.error>()
+export const exportVariable = jest.fn<typeof core.exportVariable>()
+export const getInput = jest.fn<typeof core.getInput>()
+export const getState = jest.fn<typeof core.getState>()
+export const info = jest.fn<typeof core.info>()
+export const saveState = jest.fn<typeof core.saveState>()
+export const setFailed = jest.fn<typeof core.setFailed>()
+export const setOutput = jest.fn<typeof core.setOutput>()
+export const warning = jest.fn<typeof core.warning>()