feat: add credo and dialyxir for code quality checks

[abc3]

This PR adds Credo and Dialyzer to the project and configures them in a github action

Update: Removed auto-check from the GA, as Dialyzer is unhappy

to run locally:

mix dialyzer

mix credo

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[acco]

@abc3 Thanks for this – looks like these are both setup to be optionally run, is that right? I don't think we're ready to add to our CI workflow, so it might not make sense to add them at all (because they'll just be failing)

Dialyzer
I don't think Dialyzer is worth the effort. We had "Dialyzer 0" on our last project, and I'm not convinced it was worth the ROI.

The investment:

• Time to run CI/get PRs approved
• Time invested to fix Dialyzer when it's broken in a developer environment
• Time getting nerd sniped by a Dialyzer error that a developer can't help but try to identify/fix

The return was it caught a handful of bugs. Lot of other higher yield time investments to catch bugs or improve the system!

FWIW I still have Dialyzer running in my IDE and it can be helpful when building new stuff as part of the feedback loop.

Credo
I think there's a new emerging lib that's like Styler but for Credo? Ideally, the tools fix the issues vs tell us they are there.

[abc3]

@acco Yeah, they are run optionally. I added them because of this comment. Dialyzer detects an issue with :eredis.qp. However, feel free to reject this PR if you don't think it's worth it.