Convert to ESM.

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

jest.config.js

@@ -1,14 +1,29 @@
-module.exports = {
+// See: https://jestjs.io/docs/configuration
+
+/** @type {import('ts-jest').JestConfigWithTsJest} **/
+export default {
   clearMocks: true,
   collectCoverage: true,
   collectCoverageFrom: ['./src/**'],
   coverageDirectory: './coverage',
   coveragePathIgnorePatterns: ['/node_modules/', '/dist/'],
   coverageReporters: ['json-summary', 'text', 'lcov'],
-  moduleFileExtensions: ['js', 'ts'],
+  extensionsToTreatAsEsm: ['.ts'],
+  moduleFileExtensions: ['ts', 'js'],
+  preset: 'ts-jest',
+  reporters: ['default'],
+  resolver: 'ts-jest-resolver',
+  testEnvironment: 'node',
   testMatch: ['**/*.test.ts'],
+  testPathIgnorePatterns: ['/dist/', '/node_modules/'],
   transform: {
-    '^.+\\.ts$': 'ts-jest'
+    '^.+\\.ts$': [
+      'ts-jest',
+      {
+        tsconfig: 'tsconfig.eslint.json',
+        useESM: true
+      }
+    ]
   },
   verbose: true
 }