ENH: add 'label' entity to anatomical masks in nipreps.json

[mnoergaard]

This PR addresses issue #972 by adding the 'label' entity to anatomical masks, as defined in the common BIDS derivatives specification (https://bids-specification.readthedocs.io/en/stable/derivatives/imaging.html#masks).

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 71.54%. Comparing base (b746cba) to head (ef8975b).
⚠️ Report is 2 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #973   +/-   ##
=======================================
  Coverage   71.54%   71.54%           
=======================================
  Files          89       89           
  Lines        8653     8653           
  Branches     1077     1077           
=======================================
  Hits         6191     6191           
  Misses       2230     2230           
  Partials      232      232           

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