Allow reading alternative nasion label (Nz) from snirf file

[Aniketsy]

Fixes #13615

[larsoner]

Failure looks related, test might need to be adjusted (maybe this dataset goes from no dig in head coords to having some now perhaps?)

___________________ test_snirf_standard[fname3-0-True-True] ___________________
mne\io\snirf\tests\test_snirf.py:506: in test_snirf_standard
    _test_raw_reader(
mne\io\tests\test_raw.py:530: in _test_raw_reader
    assert len(eeg_dig_head) == len(eeg_dig)
E   assert 0 == 297
E    +  where 0 = len([])
E    +  and   297 = len([<DigPoint |     EEG #1 : (0.2, -120.5, -25.8) mm   : unknown frame>, <DigPoint |     EEG #2 : (-0.5, -8.4, 101.4) mm    : unknown frame>, ...)

[Aniketsy]

@larsoner thanks! I've applied the changes. I haven’t yet set up the environment locally to run this end to end, but I’ll do that next. please let me know if anything needs further improvement.