Convert to ESM.

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

__fixtures__/cache.ts

@@ -0,0 +1,7 @@
+import * as cache from '@actions/cache'
+import { jest } from '@jest/globals'
+
+export const ReserveCacheError = cache.ReserveCacheError
+export const restoreCache = jest.fn<typeof cache.restoreCache>()
+export const saveCache = jest.fn<typeof cache.saveCache>()
+export const ValidationError = cache.ValidationError