Investigate FASTJSON2 vs Gson for property de-/serialization

[PimvanderLoos]

The current property setup uses FASTJSON2 for de-/serialization of property values.

Initially, FASTJSON2 was selected because it was faster and purely used internally. However, given that we now expose (parts of) our serialization system to external plugins, we should re-evaluate whether this is still the right library for us.

Serialization performance is no longer the primary factor, as it should also be easy to use for external plugins.

While FASTJSON2 is pretty easy to use overall, it has two significant disadvantages:

1. The general Minecraft plugin ecosystem uses Gson as it is packaged with Spigot by default.
2. FASTJSON2's documentation is awful and there is a severe lack of examples available, which isn't the case for Gson.

This is a follow-up to PR #896