build: Enable ESM support

Configure TypeScript and Jest to enable import of ECMAScript modules.
This enables to un-mock `react-markdown` and test functionality
surrounding its usage in the source.

jest-setup.ts

@@ -3,11 +3,15 @@ import '@testing-library/jest-dom';
 // used by `@php-wasm/universal` and it's not available in the Jest environment.
 // eslint-disable-next-line import/no-unresolved
 import 'web-streams-polyfill/polyfill';
+import { jest } from '@jest/globals';
 import nock from 'nock';
+import ResizeObserverPolyfill from 'resize-observer-polyfill';
 
 if ( typeof window !== 'undefined' ) {
 	// The ipcListener global is usually defined in preload.ts
-	window.ipcListener = { subscribe: jest.fn() };
+	window.ipcListener = {
+		subscribe: jest.fn( () => jest.fn() ),
+	};
 
 	// Mock `matchMedia` as it's not implemented in JSDOM
 	// Reference: https://jestjs.io/docs/manual-mocks#mocking-methods-which-are-not-implemented-in-jsdom
@@ -35,7 +39,7 @@ jest.mock( './src/hooks/use-offline', () => ( {
 } ) );
 
 jest.mock( 'strip-ansi', () => ( {
-	default: jest.fn().mockImplementation( ( str: string ) => str ),
+	default: jest.fn( ( str: string ) => str ),
 } ) );
 
-global.ResizeObserver = require( 'resize-observer-polyfill' );
+global.ResizeObserver = ResizeObserverPolyfill;

jest.config.ts

@@ -1,12 +1,14 @@
 module.exports = {
 	roots: [ '<rootDir>/src' ],
+	preset: 'ts-jest/presets/default-esm',
 	transform: {
 		'^.+\\.tsx?$': [
 			'ts-jest',
 			{
 				diagnostics: {
 					exclude: [ '**/vendor/wp-now/**/*' ],
 				},
+				useESM: true,
 			},
 		],
 	},

package.json

@@ -24,8 +24,8 @@
 		"publish": "electron-forge publish",
 		"lint": "eslint --ext .ts,.tsx,.js,.jsx,.mjs .",
 		"format": "prettier . --write",
-		"test": "jest",
-		"test:watch": "jest --watch",
+		"test": "node --experimental-vm-modules --trace-warnings node_modules/jest/bin/jest.js",
+		"test:watch": "node --experimental-vm-modules node_modules/jest/bin/jest.js --watch",
 		"e2e": "npx playwright install && npx playwright test",
 		"make-pot": "wp-babel-makepot \"./src/**/*.{js,jsx,ts,tsx}\" --ignore \"**/*.d.ts\" --base \"./src\" --dir \"./out/pots\" --output \"./out/pots/bundle-strings.pot\""
 	},

tsconfig.json

@@ -2,7 +2,7 @@
 	"compilerOptions": {
 		"target": "ES6",
 		"allowJs": true,
-		"module": "commonjs",
+		"module": "ESNext",
 		"skipLibCheck": true,
 		"esModuleInterop": true,
 		"noImplicitAny": true,