Add `pre-commit` `file-contents-sorter` as builtin hook

`file-contents-sorter` would be great to have! simple to implement: read lines, sort, write back, exit non-zero if modified. should support `--unique` and `--ignore-case` to match `pre-commit-hooks`.

```yaml
repos:
  - repo: builtin
    hooks:
      - id: file-contents-sorter
        files: ^tools\.yml$
```