Add support for smooth sort

Sorts an array using the smooth sort algorithm.

```js
const arr = [2, 5, 10, 5, 32, 6];
smooth(arr); // [2, 5, 5, 6, 10, 32]
smooth(arr, (a, b) => b - a); // [32, 10, 6, 5, 5, 2]
```

**Update README.md**.

Start by reading README.md and the repository's existing sorting implementations to locate the public entry point and naming conventions. Add smooth-sort support with the shown ascending and comparator-based descending behavior, then update README.md with the documented usage example.

Written by the indexing model from the issue text.