chore: migrate to ESM (#303)

- [Why do I need `.js`
extension?](https://stackoverflow.com/a/77150985/6512681)
- [Why setting `rootDir` in the
`tsconfig.json`?](https://stackoverflow.com/a/58941798/6512681)
- [How to ensure that we add the `.js` extension via
ESLint](https://github.com/import-js/eslint-plugin-import/blob/main/docs/rules/extensions.md#importextensions)

Fixes https://github.com/microsoft/playwright-mcp/issues/302

tests/sse.spec.ts

@@ -14,14 +14,18 @@
  * limitations under the License.
  */
 
+import url from 'node:url';
 import { spawn } from 'node:child_process';
 import path from 'node:path';
-import { test as baseTest } from './fixtures';
+import { test as baseTest } from './fixtures.js';
 import { expect } from 'playwright/test';
 
+// NOTE: Can be removed when we drop Node.js 18 support and changed to import.meta.filename.
+const __filename = url.fileURLToPath(import.meta.url);
+
 const test = baseTest.extend<{ serverEndpoint: string }>({
   serverEndpoint: async ({}, use) => {
-    const cp = spawn('node', [path.join(__dirname, '../cli.js'), '--port', '0'], { stdio: 'pipe' });
+    const cp = spawn('node', [path.join(path.dirname(__filename), '../cli.js'), '--port', '0'], { stdio: 'pipe' });
     try {
       let stdout = '';
       const url = await new Promise<string>(resolve => cp.stdout?.on('data', data => {