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!