feature: add shtfmt formatter for bash/shell scripts

[Jiehong]

Hello,

I've been using spotless as a maven plugin for a while now, and it's very nice.

The best part is its support for many languages/files.

It would be really nice if spotless would allow using shtfmt (https://github.com/mvdan/sh) to format bash/sh files with a given extension.

Would that be possible?

[nedtwigg]

A PR for this is welcome: https://github.com/diffplug/spotless/blob/main/CONTRIBUTING.md