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

src/tools/files.ts

@@ -15,7 +15,7 @@
  */
 
 import { z } from 'zod';
-import { defineTool, type ToolFactory } from './tool';
+import { defineTool, type ToolFactory } from './tool.js';
 
 const uploadFile: ToolFactory = captureSnapshot => defineTool({
   capability: 'files',