All source code files as well as README.md, .gitignore, and various configuration files have been updated with an SPDX license identifier, changing license from CC0-1.0 to MIT. The copyright year has been updated to 2023, and the copyright holder's email address has been corrected from 'winni@neessen.dev' to 'wn@neessen.dev'. An MIT license text file has been additionally added.
This commit introduces a SonarQube configuration setup, adds an .idea/.gitignore file, and expands the root .gitignore file. The SonarQube setup includes a GitHub Actions workflow and a project properties file. The .gitignore files are updated to properly ignore files for JetBrains IDEs, local testfiles, SonarQube, and others.
