Follow up to refactoring the generic utils mixin used in select-kit components in #32594.

This PR follows a different approach as the util functions related to properties here aren't easily extracted without a major change to the interface due to the dependency on this.selectKit. These util functions are instead declared on the prototype with a class decorator which ensures the same behaviour is maintained without relying on a mixin.

It's largely a lift-and-shift with some minor refactoring of the conditional logic to reduce nesting and improve readability of the functions.