Add the “allowShulkerInBackpacks” option to usefulbackpacks-common.toml

[panic08]

Detailed description of the feature

The main case is to add a special allowShulkerInBackpacks option to usefulbackpacks-common.toml

In the configuration file there is already a similar option allowStackingBackpacks which forbids to put a backpack in a backpack. However, we can still do this indirectly with shulkers. This change will make a balance when this option is set to true. If a player wants to be able to put backpacks in hackers and hackers in backpacks, he can set the option that he needs to do so himself

Verify feature request

• I have checked that this is not a duplicate feature request and the feature does not yet exist in the latest version