Add external linter

[ClFeSc]

I added an external linter that detects some issues regarding our coding standards (and some basic rules). Not covered are the following:

• Final newlines: It seems to be a little bit complicated to read a file with its final space characters. Since the refactoring fixes any such issues, I didn't include a check for it here for now. I'll look into adding it to the linter.
• Breaks in blocks, loops, etc.: I think this is a very subjective feature and can't really be checked automatically. If you have any ideas, feel free to contribute.
• Alphabetical sorting of instance / class variables: I didn't have the time yet to check for this, but I'll look into it.

I decided to split up the errors in "correctable" and "uncorrectable" issues where correctable issues may be fixed by the auto-refactoring. I decided to fail the linter any time any issue occurs, even though it's correctable since I want to encourage the use of the refactoring to improve the code quality.

Feel free to leave your thoughts on this, including further check ideas.

Note that the pipeline will currently fail as there are several issues the linter detects.

[ClFeSc]

I think I won't do much more for this respecting the time remaining.