How should pyrefly handle unions inside intersections?

### Describe the Bug

A gap in our current implementation of intersections is that if we see a union inside an intersection, we simply give up on creating the intersection: https://github.com/facebook/pyrefly/blob/03131fd1c7fea035d07a219d00c26ac793efd27b/crates/pyrefly_types/src/simplify.rs#L98-L104
We should figure out a more principled way of handling this. An obvious solution is to immediately normalize the type, but that's potentially complicated and expensive.

Note that, in practice, this currently isn't a problem because we distribute over unions first in the only place where we create intersections: https://github.com/facebook/pyrefly/blob/03131fd1c7fea035d07a219d00c26ac793efd27b/pyrefly/lib/alt/narrow.rs#L131

### Sandbox Link

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### (Only applicable for extension issues) IDE Information

_No response_