feat: Track files referenced by other files to identify orphans

[effigies]

This implements the legacy checks:

• UNUSED_STIMULUS
• SIDECAR_WITHOUT_DATAFILE

The approach is to add a .viewed boolean field on BIDSFile and trip it when either of the inheritance principle walkBack() or the expression language exists() functions finds the file.

Note that apparently we had a lot of false negatives in the legacy validator, and SIDECAR_WITHOUT_DATAFILE is causing failures of examples.

Should wait on #2064, #2066 and bids-standard/bids-examples#461.

Diff specific to this PR: bids-standard/bids-validator@2ecf188...e5aeeca

[codecov]

Codecov Report

Attention: Patch coverage is 90.00000% with 6 lines in your changes missing coverage. Please review.

Project coverage is 87.25%. Comparing base (a4e1a09) to head (73c52d1).

Files	Patch %	Lines
...ds-validator/src/validators/internal/unusedFile.ts	82.85%	6 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2057      +/-   ##
==========================================
+ Coverage   85.69%   87.25%   +1.56%     
==========================================
  Files          91      140      +49     
  Lines        3782     6788    +3006     
  Branches     1220     1597     +377     
==========================================
+ Hits         3241     5923    +2682     
- Misses        455      775     +320     
- Partials       86       90       +4     

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