Enable matching and substitution for operator evaluations

[Chessing234]

Summary

• matchWithGivenSentence previously refused operator evaluation terms.
• Recursively match operator + args (same idea as equality predicates).
• subsSentence rebuilds matched operators via operatorTerm.

Test plan

• In the interactive text, try a law that templates over an operator evaluation (e.g. cancellation-style exercises)
• Confirm equality matching still works