Merge flying entities from spawner ignoring the stack-down setting

[Aeyos]

From #620

Make flying entities spawned from a spawner merge together without needing to disable the stack-down feature. To solve this issue:

The config section had to be modified

  stack-down:
    # Should the stack-down feature be enabled on the server?
    enabled: false
    # A list of entities that will be forced to only stack down.
    # You can combine filters using "ENTITY_TYPE:SPAWN_REASON"
    stack-down-types:
    - BLAZE
    - GHAST

It'd be more intuitive if the spawner merging logic ignored this setting and merged ghasts and blazes into a single stack by default