`fs: true` does not work due to missing default export

```
[!] Error: 'default' is not exported by node_modules/rollup-plugin-node-polyfills/polyfills/buffer-es6.js, imported by node_modules/rollup-plugin-node-polyfills/polyfills/browserify-fs.js
https://rollupjs.org/guide/en/#error-name-is-not-exported-by-module
node_modules/rollup-plugin-node-polyfills/polyfills/browserify-fs.js (2:7)
1: import util$2 from 'util';
2: import buffer from 'buffer';
```

`polyfills/buffer-es6.js` is missing default export, adding this line seems to work.

```
export default { Buffer, INSPECT_MAX_BYTES, SlowBuffer, isBuffer, _kMaxLength: kMaxLength };
```

Comienza con polyfills/buffer-es6.js y el import de polyfills/browserify-fs.js que se muestra en el error de Rollup. Reproduce el fallo de compilación de `fs: true` y, a continuación, verifica que el export por defecto que falta esté presente y que el error de Rollup ya no se produzca.

Escrito por el modelo de indexación a partir del texto del issue.