Reformat shell scripts with shfmt and validate in CI

[msfjarvis]

Uses mvdan/sh to enable formatting checks in CI, and correctly fails when the codestyle is incorrect, outputting a diff with fixes.

I chose the set of options that made the most sense to me, all available switches are listed here.

[nathanchance]

Yes, the README has installation instructions it seems (or I guess you could either apply the diff or do it manually but that seems like a lot of effort :P)

[msfjarvis]

neat, so looks like we'll need to grab the diff from the github action when the presubmit check fails? Is it possible to run shfmt locally to be proactive?

I personally install shfmt through Linuxbrew but it appears the maintainer also makes binaries available in the Releases tab.