add formatter

[jase88]

Story

As a developer, I would like to have a automated formatting for the code, because it

• enforces a consistent code style
• saves time
  • not to manually adjust the formatting
  • on merge conflicts

It would also mitigate the point mentioned in the Contributing Guide, as there can be no unnecessary discussions about code formatting.

Technical details

• add a code formatter like prettier or biome with an appropiate configuration

Acceptance criteria

• an formatter is configured for the project that is ideally automatic picked up by IDEs
• CI checks if all files are properly formatted with the tool