cspell: add nifti in dictionnary

nifti is a file format which will be supported, and needs to be added in sight cspell dictionnary

add nifti in the dictionary

nifti should no more be treated as error in the cspell treatment

add nifti in hooks/.cspell/sight-dict.txt

To upload designs, you'll need to enable LFS and have an admin enable hashed storage. More information