fix: integrate dev tools and fix crusial errors and warnings

[phts]

Integrated for whole repo, but enabled only for "spotify". Feel free to enable it for other repos in future in .eslintignore and .prettierignore files.

Easier to review commit-by-commit, which represent independent step

Commit 3024fce contains fixes of crucial errors which were found by eslint (which caused crashes and misbehavior)

[volumio]

Thanks for this, but I see that many global variables are removed (like selectedBitrate). This huge diff on index.js requires careful testing and it's too much for me to read and validate without very extensive testing.

IMHO:

• Let's just lint and check everything besides index.js
• On index, let's just fix errors (like done in 3024fce)

Otherwise it's really hard for me to check and get it in

[phts]

Those globals are not used and not referenced anyhow. But I revered them

No sense to integrate without any automation, and keeping everything unchanged. Based on my experience, in this case it will never be fixed.

All those formatting and recommended codestyle is performed automatically on every commit. And that makes code style consistent for all contributors and prevent most typos and mistakes. This autofix is save and used by millions devs every day.

I couldn't fix those crucial mistakes without autofix by eslint all autofixable errors. Otherwise whole file would be red in my IDE, or I would to turn off all basic rules to ignore such warnings, which also makes whole eslint stuff almost useless.

Additionally I fixed warnings (like unused variable) manually and also is safe.

And better to review this commit-by-commit, those describe single independent change each other.

Usually, you can rebase all your future development onto this branch, and you will use all benefits of linting and test everything during development.

[phts]

@volumio

integrated github actions with running lint job on every merge request