Add override for Block.entityInside

[Jishuna]

Description

Allows customizing what happens to an entity when they are inside a custom block.
This allows for the creation of things like poison ivy or toxic liquids.

Example Script

// Kills any entity that steps into the block.
StartupEvents.registry('block', event => {
     event.create('test')
     .noCollision()
     .entityInside(callback => {
        callback.entity.setHealth(0);
     });
});

Other details

[ChiefArug]

Good idea for another callback!

[ChiefArug]

I don't like this inheritance tree. It makes no sense conceptually, and if Mojang add a new parameter to stepped on then it will need reworking. Maybe instead rename EntitySteppedOnBlockCallback to a more generic 'EntityBlockCallback' and use that directly in both places?

[ChiefArug]

This class doesn't need to exist if all it doesn't do anything its superclass does.

[ChiefArug]

Does this hold even when an entity is suffocating in the block?