Add expression interpreter

[ubdbra001]

Here's the initial attempt at adding all the parts of the expression language interpreter.

These successfully cover the full range of tests in the bidsschematools expression tests.
Two things that still need addressing:

• exists passes the tests but needs more work to actually work as described in the schema tools docs
• I'm not sure yet how to integrate them with the Context

[codecov]

Codecov Report

❌ Patch coverage is 25.50336% with 111 lines in your changes missing coverage. Please review.
✅ Project coverage is 77.68%. Comparing base (878d57d) to head (ca15030).

❌ Your patch check has failed because the patch coverage (25.50%) is below the target coverage (80.00%). You can increase the patch coverage or adjust the target coverage.
❌ Your project check has failed because the head coverage (77.68%) is below the target coverage (80.00%). You can increase the head coverage or adjust the target coverage.

❗ There is a different number of reports uploaded between BASE (878d57d) and HEAD (ca15030). Click for more details.

HEAD has 15 uploads less than BASE

Flag	BASE (878d57d)	HEAD (ca15030)
	16	1

Additional details and impacted files

@@             Coverage Diff             @@
##             main      #71       +/-   ##
===========================================
- Coverage   90.89%   77.68%   -13.21%     
===========================================
  Files          13       14        +1     
  Lines         846      995      +149     
  Branches      124      167       +43     
===========================================
+ Hits          769      773        +4     
- Misses         47      194      +147     
+ Partials       30       28        -2     

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