Improve Typings

[KaiVolland]

Currently there are some minor issue with the types of the geostyler-style. We should check if we can improve these types without breaking changes.

1. GeoStylerFunction is a union type of all functions. This leads to issues with the (currently two) functions that don't have args. In particular TypesScript says that a GeoStylerFunction has no property args unless you exclude the functions without args. We should avoid adding an empty array as args but find a TypeScript way for this.

2. PropertyType includes unknown. This translates to PropertyType = unknown which is obviously not perfect.