fix: Improve type output for narrowed interfaces (intersect members of unions instead)

[kitten]

Summary

I noticed that the types on interfaces and unions could become quite complex and the type output wasn't nice to read.

Instead of asking TypeScript to narrow using an intersection, we can instead manually narrow any union, since they're expanded in extends types.

Note: I'm not 100% sure what the type performance impact of this change might be, but the benchmarks indicate that it's negligible.

Follow-up to #368
But issue is also visible before the above PR was merged. It was just rarer

Set of changes

• Expand union types before intersecting

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