ci: enable linters on changed code

[joka134]

This PR adds a new golangci-lint config that will help us to keep a more consistent style of code, which in the end helps us in code reviews and maintenance.
For now we still use the default settings when linting the whole repository (using .golangci-tmp.yml) and our target config (.golangci.yml) is only used on changed files (not only changed lines).

The target config is also the default config when using golangci during development. So I highly recommend setting golangci-lint v2 as default linter in vscode.

I also fixed two files as an example. I don't think fixing the findings takes too much time.
It's usually something like:

• adding a package comment (default godoc style)
• adding comments to exported functions (also godoc)
• using a consistent whitespace style (not the one we all prefer probably, but at least the same for all 😄 )

The new linters in addition to the default settings are:

• https://revive.run/
• https://github.com/bombsimon/wsl
• https://github.com/tomarrell/wrapcheck