whenExpression: when using expr instead of govaluate

[alexec]

Summary

We don't like govaluate, we want to get off it, it is confusing to users:

• Confusingly, when _kind of_ supports expr expressions #7576
• Confusing expression evaluation behavior #7509
• "expression" syntax should be the same across all of the places it is used #7089
• Use either expr or govaluate consistently, not both #6823
• Expand documentation on conditional expressions (govaluate) #7382

---

Message from the maintainers:

Love this enhancement proposal? Give it a 👍. We prioritise the proposals with the most 👍.

[EzrealK]

#7413