Allow military NVGs to attach to combat exoskeleton helmets

[Regilith]

Summary

Features "Allow military night vision goggles to attach to combat exoskeleton head armors"

Purpose of change

Implements #75978

Military night vision goggles were recently made to only be usable when attached to military helmets with the proper mounts. Considering exoskeletons were developed by the military and made to be modular, it seems like an oversight that the suits would not be able to utilize this technology to allow soldiers to see in the dark, without relying on CBMs.

Describe the solution

Added MODULE_HOLDER to the base exoskeleton, copy/pasted the "HELMET_FRONT_ATTACHMENT" pocket from the army helmet to the heavy exoskeleton head armor, which all other exoskeleton helmets "copy-from".

Describe alternatives you've considered

Create a proprietary solution for the exoskeletons, make a rare drop somewhere. Seems like an unnecessary duplication of code though, and would likely necessitate the creation of a unique flag just for that item. Allowing players to use NVGs with exoskeleton armor doesn't seem like something that needs to be earned, since exoskeletons are locked behind one of the toughest bosses in the game already.

Make the survivor custom-create a bracket to allow the helmet to accept NVGs. This doesn't really help explain the issue as to why the military would not make the exoskeletons have NVG functionality in the first place.

Make the NVG effect built-in to the exoskeleton itself, but the other power armors already do that, and I think to better fit the flavor of the combat exoskeleton we should lean on the modular philosophy of the exoskeleton and use pre-existing NVGs.

Testing

Copy/pasted the edited json files into a freshly-installed latest experimental. Inserted all three military NVGs into four different exoskeleton helmet armors, NVG effect was applied each time.

Additional context

I also considered adding flavor text to all the helmets, mentioning the addition of the new pocket. Since it did not mention the pre-existing pocket for a headlamp though, and to keep this PR small, I opted not to.