Schema: add validation for datasource property referentialIntegrity & relationMode, so only one can be set.

[Jolg42]

Currently, the following is valid

datasource db {
  provider = "sqlite"
  url = "..."
  relationMode = "prisma" // we take relationMode in priority
  referentialIntegrity = "foreignKeys" // is ignored
}

We want to make it invalid, so only relationMode or referentialIntegrity can be used.

Note that we also want to deprecate referentialIntegrity property, see #15689