fix: Add type annotations, fix a bug in loading schema path

[effigies]

Adding type checking in bids-standard/python-validator#46, we found that some bidsschematools modules we were using were under-annotated.

This adds annotations for the main schema API and the utilities module.

Two bugs were caught by mypy:

• bst.utils.get_bundled_schema_path called a non-existent function to get the schema path
• A few functions would not handle non-Path path-likes well.

[codecov]

Codecov Report

❌ Patch coverage is 94.11765% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 82.58%. Comparing base (953e550) to head (1ecb936).

Files with missing lines	Patch %	Lines
tools/schemacode/src/bidsschematools/schema.py	92.00%	2 Missing ⚠️
.../schemacode/src/bidsschematools/types/namespace.py	96.77%	1 Missing ⚠️
tools/schemacode/src/bidsschematools/utils.py	90.00%	1 Missing ⚠️

Additional details and impacted files

@@               Coverage Diff                @@
##           maint/1.10.1    #2264      +/-   ##
================================================
- Coverage         82.70%   82.58%   -0.12%     
================================================
  Files                20       20              
  Lines              1659     1665       +6     
================================================
+ Hits               1372     1375       +3     
- Misses              287      290       +3     

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