Switch over to Error Prone (now with NullAway!)

There is one major change: we now have a separate artifact for the annotation processor.

As for NullAway, we are currently exempting the clientbound join game/respawn packets. They are ugly and need to be refactored.

settings.gradle

@@ -2,8 +2,10 @@ rootProject.name = 'velocity'
 include(
         'api',
         'proxy',
-        'native'
+        'native',
+        'annotation-processor'
 )
+findProject(':annotation-processor')?.name = 'velocity-annotation-processor'
 findProject(':api')?.name = 'velocity-api'
 findProject(':proxy')?.name = 'velocity-proxy'
 findProject(':native')?.name = 'velocity-native'