chore: restructure 'platform' package and add 'jre' package

[skaldarnar]

Contains

Since we want to manage JREs for different platforms, the respective code needs to become a bit more elaborate. Therefore, introduce a new platform package with the Platform utility class and OS and Arch enums.

In addition, start working on jre-related classes to define and handle JREArtefacts to be downloaded and managed.

How to test

Ensure that the launcher is still working as before (no new features).

Outstanding before merging

Not yet ready for merging.