date field `orderBy` doesn't compare different types

[electrovir]

Prisma allows creating date fields with a string value or a Date instance. If a mix of Date instances and strings are used with Prismock, however, orderBy don't properly compare the two.

Repro steps:

1. create a schema with a date field
2. create some rows with Date instances
3. create some rows with (ISO formatted) date strings that are earlier than the dates in part 2
4. try a .findMany operation with orderBy
5. notice that rows from part 3 are incorrectly ordered after the rows from part 2