There are a few motivations here:
1) Less boilerplate code (this can be written inline)
2) It's possible to provide multiple constructors for different packet variations to reduce the chance of errors.
3) It makes things catch fire on updates in ways that static analysers can understand.
This has been a pain point for a long time due to the misleading nature of the name "level". It's also confusing when trying to do things like getting the XP level of the player or such, and also does not translate well to other languages.
This transition was already executed on the UI some time ago (language strings) and now it's time for the same change to occur on the API.
This will burn a lot of plugins, but they'll acclimatize. Despite the scary size of this PR, there isn't actually so many changes to make. Most of this came from renaming `Position->getLevel()` to `Position->getWorld()`, or cosmetic changes like changing variable names or doc comments.
