Smoothened out a few bugs and done some improvements

Features added:
- Added AdvancementManager#loadCustomProgress(Player player, String json) to load all advancements (not taking namespace into account) since you can also get the save string for all advancements
- Added AdvancementManager#loadCustomProgress(Player player, String json, String namepace) to support only loading a set namespace (as you can also save a set namespace)
- Renamed AdvancementManager#loadProgress(Player player, String json, Advancement... advancementsLoaded) => AdvancementManager#loadCustomProgress(Player player, String json, Advancement... advancementsLoaded) to allow adding the above methods and having them all have the same method name without them becoming ambiguous with other load methods

Bugs fixed:
- Fixed bugs regarding getting/saving the progress json
- Only advancements loaded from the default folder are now in the advancement map (as intended)
- Minor bug fixes