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/headed.spec.ts

@@ -14,7 +14,7 @@
  * limitations under the License.
  */
 
-import { test, expect } from './fixtures';
+import { test, expect } from './fixtures.js';
 
 for (const mcpHeadless of [false, true]) {
   test.describe(`mcpHeadless: ${mcpHeadless}`, () => {