`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 };
```

Start with polyfills/buffer-es6.js and the import from polyfills/browserify-fs.js shown in the Rollup error. Reproduce the `fs: true` build failure, then verify that the missing default export is present and that the Rollup error no longer occurs.

Scritto dal modello di indicizzazione a partire dal testo della issue.