[FIX] Allow (but discourage) task entity for coordsystem.json

[effigies]

We've run into a situation where the regular validator regular expressions were written more laxly than the specification, leading to datasets that use entities that are against the spec but perfectly interpretable. In particular, the task entity has been permitted for coordsystem.json across EEG, iEEG, MEG and NIRS.

Following the pattern established in #1722, instead of marking these datasets invalid with a schema-based validator, we relax the spec but add warnings.

cc @bids-standard/raw-electrophys and @rob-luke @lpollonini (we should make a NIRS team...).

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 88.06%. Comparing base (0e506f0) to head (c0915eb).

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1888   +/-   ##
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  Coverage   88.06%   88.06%           
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  Files          16       16           
  Lines        1391     1391           
=======================================
  Hits         1225     1225           
  Misses        166      166           

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