Use `Intl.Segmenter` when available?

Investigate using https://github.com/tc39/proposal-intl-segmenter when it becomes available.

```js
new Intl.Segmenter('en', {granularity: 'grapheme'});
```

Related issue: https://github.com/sindresorhus/string-length/issues/14

We can use it when targeting Node.js 16.