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 https://github.com/bids-standard/bids-examples/pull/461.

Diff specific to this PR: https://github.com/bids-standard/bids-validator/compare/2ecf188...e5aeeca

[codecov[bot]]

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 :warning:

Additional details and impacted files

```diff @@ 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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