Refactoring TODOs

Due to the growing complexity (expressions, async handling, …) the code is getting messy so we need to do some refactoring.
Suggested TODOs are:

- [ ] Split code into more files (at least read and write)
- [ ] Split `mapboxLayersToGeoStylerRules` into mulitple functions
- [ ] Cleanup handling of metadata (currently fragile state manipulation)

Feel free to add or suggest more.