Epic: Support for multiSchema (GA) in Language-Tools

[eviefp]

We should support the following scenarios

Multi-Schema GA

• [x] #1331
• [x] #1329
• [x] https://github.com/prisma/prisma/issues/16920
• [x] https://github.com/prisma/prisma/issues/16957
• [x] #1332

Bonuses

• [x] #1333
• [ ] #1339

[Druue]

We shouldn't error just when the preview feature is enabled as post-ga this will effectively "always be enabled"