The /fsh-tank folder is a temporary storage location for FHIR Shorthand (FSH) files used in the generation of this IG.
The entire IG is not (yet!) built in FSH, but some resources (primarily terminology) are built this way for ease-of-development.
The npm script sushi first runs Sushi and then converts its output JSON and stores it into /input alongside the rest of the IG source.
The IG publisher ignores the /fsh-tank folder
None of the /fsh-tank or package.json scripts are necessary for working in this IG; they're just helpful.