Add a way of testing built-in and util types in Redwood.

Worth exploring: https://github.com/SamVerschueren/tsd

Great types to start with:

1. Route parser
Move this type into @redwoodjs/router instead of the generated file, and import it in the generated file. This'll let us test it using tsd

2. MergePrismaTypes
Add type tests for this, to prevent regressions and allow better debugging