rf(context): Refactor inheritance principle to walk from leaf to root

[effigies]

This PR proposes a single implementation of the inheritance principle.

• BIDSFile gains a .parent reference to a FileTree instance. For the moment it was simplest to treat it as optional, because the browser implementation constructs the BIDSFile objects before the containing FileTree objects.
• src/files/inheritance.ts exposes a walkBack generator function that finds at most one matching file in each level. If inherit = False, it only looks at the leaf level.
• Loading sidecars and building associations is trivial from here.

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 88.23529% with 8 lines in your changes missing coverage. Please review.

Project coverage is 87.09%. Comparing base (f4de42a) to head (8f682d0). Report is 2 commits behind head on master.

Files	Patch %	Lines
bids-validator/src/files/inheritance.ts	81.57%	7 Missing :warning:
bids-validator/src/schema/context.ts	87.50%	1 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #2035 +/- ## ========================================== + Coverage 85.75% 87.09% +1.33% ========================================== Files 91 134 +43 Lines 3785 6402 +2617 Branches 1218 1543 +325 ========================================== + Hits 3246 5576 +2330 - Misses 453 736 +283 - Partials 86 90 +4 ```

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