feat: Add unique() function to schema expression language

[effigies]

Companion to bids-standard/bids-validator#261.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 82.71%. Comparing base (fe39a22) to head (d325a9e).
⚠️ Report is 79 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #2214   +/-   ##
=======================================
  Coverage   82.71%   82.71%           
=======================================
  Files          20       20           
  Lines        1608     1608           
=======================================
  Hits         1330     1330           
  Misses        278      278           

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[effigies]

@drammock Would you mind having a look?

[drammock]

LGTM, just one question (see below). Thanks for taking this off my plate! Was hoping to get to it this week but always glad to review someone else's code instead