[actions] Add shellcheck workflow

[reasonablytall]

This adds a github workflow that runs shellcheck on nvm.sh and install.sh. nvm.sh is tested with each shell in [bash, sh, dash, ksh] and install.sh is tested with just bash. I think submitting this PR should run the workflow, but just in case, here is an example of this change running on my fork.

Unfortunately shellcheck does not support zsh, which is why it's omitted from the matrix.

I ignored SC1001 in nvm.sh because there are three instances where \command ... is used (i'm guessing) to suppress aliases. More recent versions of shellcheck automatically ignore these instances, but not the older version available on the ubuntu-based github runners. Alternatively, I could ignore the specific lines in nvm.sh rather than globally ignoring SC1001, or I could figure out a way to manually install a more recent version of shellcheck. Let me know!

[reasonablytall]

Hmm looking at the list of checks created the naming looks a little messy, let me know if you have any preferences with those!

Also, maybe I should make checking install.sh a separate job, rather than a subtle part of the bash run?

[ljharb]

Yes, it should be a separate job in the matrix.

It'd be ideal to find a way to get the latest shellcheck; i rely on the things the latest version checks.

[ljharb]

k, fixed the matrix, the only thing is that this is v0.4.6, and shellcheck's up to v0.7.1. is brew available in actions? if so, brew install shellcheck would work; otherwise maybe we need to use a custom apt repo?

[reasonablytall]

Brew does work! Thanks for the matrix changes too; I wouldn't have thought of doing it that way :)

I removed the ignore SC1001 declaration as well, now that we have the most recent version of shellcheck.

[ljharb]

woot, looks great!