Fix Hard Dependency on MekEng

[IntegerLimit]

This PR fixes NAE2 having a hard dependency on Mekanism Energistics (and thus Mekanism).

This was due to two issues:

• The importing of the ItemAndBlocks class from MekEng in the Items registration class, fixed by moving the class usage into DenseGasCell
• The usage of DenseGasCell and VoidGasCell in registration, even when MekEng isn't loaded, as that causes crashes (even when the class usage is in a lambda (most likely due to the classes containing generics with and methods returning MekEng classes)