Fix sorting by multiple values

[timleslie]

Prisma no longer (if it ever? 🤔 ) accepts an object with multiple values for sorting. I've added tests to exercise this feature and to avoid any future regressions.

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Latest commit: 41b317c

The changes in this PR will be included in the next version bump.

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Name	Type
@keystone-next/adapter-prisma-legacy	Patch

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