Support quickly clearing the entire world

[RobertasJ]

I'm using this for testing terrain gen and it would be nice if there as a more performant way to clear all the voxels, right now it takes a while for it to clear all the changes since there are a lot of blocks in bigger areas.

[nathanaelneveux]

I use this little function to reset the all the chunks and it's pretty fast:

fn mark_chunks_dirty_on_change(
    mut events: MessageReader<VoxelLabSettingsChanged>,
    mut commands: Commands,
    chunks: Query<Entity, With<Chunk<VoxelLabWorld>>>,
) {
    if events.read().next().is_none() {
        return;
    }

    for entity in chunks.iter() {
        commands.entity(entity).insert(NeedsDespawn);
    }
}

VoxelLabSettingsChanged is just an event I fire whenever I change setting in a little egui interface I made.