Preserve player-set side effects

[me4502]

More a test PR to see how it behaves to see if we really want to do it, but it fixes https://github.com/EngineHub/WorldEdit/issues/2293.

As it's a class it requires a custom serializer/deserializer, which means if we were to ever add another session store type it'd need to be included there too.

Biggest caveat from testing is that once something is set in there, there's no way to set it back to "default". We're storing the list of side effects that differ from default in the SideEffectSet