feat: Begin constructing context using bidsschematools types

[effigies]

Builds on bids-standard/bids-specification#2133. Replaces #24.

This is not complete, but should be a good starting point. Apologies for the delay.

[codecov]

Codecov Report

❌ Patch coverage is 66.02564% with 53 lines in your changes missing coverage. Please review.
✅ Project coverage is 84.94%. Comparing base (d9ebe1f) to head (891d403).

Files with missing lines	Patch %	Lines
src/bids_validator/context.py	59.23%	45 Missing and 8 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #26      +/-   ##
==========================================
- Coverage   91.62%   84.94%   -6.68%     
==========================================
  Files          10       12       +2     
  Lines         442      598     +156     
  Branches       65       95      +30     
==========================================
+ Hits          405      508     +103     
- Misses         23       68      +45     
- Partials       14       22       +8     

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