feat: Add Context type generators from schema

[effigies]

Replaces #2. Still needs tests.

I got distracted on this by some bidsschematools refactorings. As it stands, attempting to import bids_validator.types.context takes some 500-600ms, around 80% of that is spent loading and dereferencing the YAML schema, and another 10% on loading the jsonschema package.

• rf(schemacode): Update packaging, minimum Python, resource loading bids-specification#2109 made jsonschema an optional dependency.
• feat: Compile schema.json and bundle in Python package bids-specification#2115 will reduce the total import time to about 35ms, including class generation.

In the long term, I would like to move this into bidsschematools, and generate static types on build, rather than at runtime, but this should let us move ahead.

[codecov]

Codecov Report

Attention: Patch coverage is 76.66667% with 21 lines in your changes missing coverage. Please review.

Project coverage is 89.38%. Comparing base (7b8b72a) to head (67bc50f).

Files with missing lines	Patch %	Lines
src/bids_validator/types/_context.py	69.11%	17 Missing and 4 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #24      +/-   ##
==========================================
- Coverage   92.16%   89.38%   -2.79%     
==========================================
  Files           9       13       +4     
  Lines         434      518      +84     
  Branches       64       81      +17     
==========================================
+ Hits          400      463      +63     
- Misses         21       38      +17     
- Partials       13       17       +4     

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