Allow to map Kotlin Datetime type to custom datetime type in js

[Legion2]

In JS there are many different builtin and libraries for representing and processing datetimes. In Kotlin the go to way is to use the official kotlinx-datetime library.
To keep the toolkit agnostic we can not enforce a single mapping and need to make it configurable. At first we will provide two mappings:

1. to String, just map all datetime types to their string representation in js
2. to js Date