Add luacheck action and fix linting warnings

[Omikhleia]

Greetings,
This PR is a proposal

• fixing several linting warnings or errors
• adding a GitHub action (= CI pipeline) to the repository, so that to ensure linting checks are performed automatically when changed are pushed.

I fixed here the most "obvious" warnings/errors, ensured qrtest.lua still reported no error, and also checked a few QR codes I had previously generated with qrcode.lua were unchanged.

As of this initial PR, though, I have left two warnings unaddressed (in the same portion of code): I am unsure what the loop there does exactly and whether this might hint toward some small logic error possibly - So I preferred not silencing them. Please feel free to check and comment.

(for that reason, I haven't proposed adding a Luacheck badge to the README, since it still fails for now)

[Omikhleia]

I could have added a link to the above-mentioned unaddressed lint warnings:
https://github.com/Omikhleia/luaqrcode/actions/runs/3664504028/jobs/6194957626

[pgundlach]

Thank you very much. I will take a look at it and report back.

[pgundlach]

The changes look good to me.

[pgundlach]

I hope I have merged this correctly. I am not an expert using GitHub and its actions. Thank you very much for your work!