add `xdg-open` to module exports

We bundle this package with `esbuild`. It will be usefull to add `xdg-open` to module exports so we can resolve it's path and copy to `build` folder. Usage example: ```js import path from 'node:path'; import { createRequire } from 'node:module'; import fs from 'node:fs/promises'; const require = createRequire(import.meta.url); await fs.copyFile( require.resolve('open/xdg-open'), path.join('build', 'xdg-open') ); ```