SCHEMA: Enable structured Level descriptions for TSV columns

[effigies]

Follow-up to #1603. Needed to support levels with schema validator.

xref bids-standard/bids-examples#413

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (47d912d) 87.83% compared to head (355ae6e) 87.83%.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1648   +/-   ##
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  Coverage   87.83%   87.83%           
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  Files          16       16           
  Lines        1356     1356           
=======================================
  Hits         1191     1191           
  Misses        165      165           

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[sappelhoff]

Thanks @effigies

[sappelhoff]

we will potentially add RotationRule and so on from:

• [ENH] Add Level objects to channels.json for motion #1591

like we did for the legacy validator in:

• https://github.com/bids-standard/bids-validator/pull/1839

see also:

• https://github.com/bids-standard/bids-validator/pull/1837