Use optional client output location when it exists

[dustin-engstrom]

The import path in the generated output assumes the Prisma client will always exist in the default location. But the Prisma client config allows users to specify a different output location:

generator client {
  provider = "prisma-client-js"
  output   = "../src/generated/client"
}

This change checks for the optional output value and uses it in the generated import path instead if it exists. Otherwise it defaults to the current behavior.

[luisrudge]

Thanks for the PR! Released in 0.13.0