rf(context): Move file tree, schema, and issues into context.dataset

[effigies]

Dataset issues sensibly accumulate on the long-lived dataset object, instead of the ephemeral context object. dataset.tree is defined by the schema, and also makes sense there.

I think walk.ts demonstrates this well. The dataset context provides the files to be iterated over, and a new context can be generated with just the current file and the dataset context.

There's still room to pre-allocate a context.subject object and pass that through, but this is enough of a change for one PR, IMO.

This will help focus #2057 on the specific changes needed for those issues.

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 79.41176% with 21 lines in your changes missing coverage. Please review.

Project coverage is 87.14%. Comparing base (12f40bb) to head (e710edd). Report is 1 commits behind head on master.

Files	Patch %	Lines
bids-validator/src/schema/applyRules.ts	40.00%	9 Missing :warning:
bids-validator/src/schema/context.ts	86.20%	8 Missing :warning:
bids-validator/src/validators/filenameValidate.ts	72.72%	3 Missing :warning:
bids-validator/src/validators/bids.ts	87.50%	1 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #2066 +/- ## ========================================== + Coverage 85.69% 87.14% +1.44% ========================================== Files 91 139 +48 Lines 3782 6689 +2907 Branches 1220 1579 +359 ========================================== + Hits 3241 5829 +2588 - Misses 455 770 +315 - Partials 86 90 +4 ```

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