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

eslint.config.mjs

@@ -33,6 +33,7 @@ const plugins = {
 };
 
 export const baseRules = {
+  "import/extensions": ["error", "ignorePackages", {ts: "always"}],
   "@typescript-eslint/no-floating-promises": "error",
   "@typescript-eslint/no-unused-vars": [
     2,