Add utilities for creating optional types#10

In the same way as in Prisma we have `createdAt DateTime @default(now())`, we should have in Pentagon something along the lines of: ```typescript const User = z.object({ createdAt: default(z.date(), () => new Date()), id: default(z.string(), () => crypto.randomUUID()) // or createdAt: default(z.date(), "now"), id: default(z.string(), "uuid") }); ```