[ENH] Provide baseline definition for behavioral files

[effigies]

Part two of resolving https://github.com/bids-standard/bids-validator/issues/1376. Also provides a starting point for behavioral files, instead of being purely free-form. If someone's going to use columns that are defined for events.tsv, they should mean the same thing in beh.tsv.

[Remi-Gau]

good with me

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 87.23%. Comparing base (183499c) to head (63ef5c8).
Report is 105 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1907   +/-   ##
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  Coverage   87.23%   87.23%           
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  Files          16       16           
  Lines        1410     1410           
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  Hits         1230     1230           
  Misses        180      180           

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