[1.20.4] Add BlockDropsEvent

[Tanticle]

This PR adds the BlockDropsEvent, which fires after the drops for a block have been determined but before they have been added to the world.
This allows for manipulation of the item entities, which is not possible when using loot modifiers (they can only change the list of item stacks).

Cancellation of the event causes no items to be dropped, but the block will remain destroyed/replaced by air. During the next breaking change window, we can also migrate the XP logic from BreakEvent to this event, as this is hooked in where XP is normally dropped, which will reduce patch overhead.

This PR also adds a breakBlock method to the GameTest helper to simulate breaking blocks with an entity and tool within a game test.

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[neoforged-pr-publishing]

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[Shadows-of-Fire]

There is value in moving the XP logic completely out of BreakEvent and into this new event, but that would be a breaking change, so we can hold off on that for now.

stale

[Shadows-of-Fire]

1.20.4 is now in feature freeze, so this will be pushed to 1.20.5; though it works out well enough, since the XP changes can be made in one pass.