criteria: add split_criteria, raising CriteriaSyntaxError on unbalanced brackets

[m-messer]

Stacked on #5: merge that first, and this PR's base will then switch to main.

Moves split_criteria out of compareExpressions' evaluation.py into lambdafeedback.criteria.

Changes

• split_criteria(criteria) in the new split.py splits on the commas outside (), [] and {}, and strips each criterion. Each kind of bracket is counted on its own.
• New CriteriaSyntaxError (a CriteriaError and ValueError) for a malformed criteria string.
• Behaviour change (CHANGELOG): a bracket closed before it is opened, or left open, now raises CriteriaSyntaxError. The old version silently stopped splitting at the first such bracket.

Downstream
compareExpressions can import this in place of its own copy. Criteria with unbalanced brackets will now raise instead of being split partially.

Verification

• ruff, format and mypy are clean, and all four test suites pass.
• New tests cover nested and mixed brackets, whitespace, a trailing comma, the empty string, and each unbalanced case.

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