PostgreSQL only: Add autocompletion in the datasource block for extensions when postgresqlExtensions preview feature is set

[Jolg42]

Problem

There is no autocompletion for the datasource property extensions

Suggested solution

if the provider is postgres / postgresql
If the postgresqlExtensions preview feature is set

We can suggest extensions = []

Inside [], should we suggest anything?
We could have a hard-coded list to suggest like postgis, citext, pg_tgrm, pgcrypto...?
Note uuid-ossp needs to be suggested as uuidOssp(map: "uuid-ossp")

Docs: https://www.prisma.io/docs/concepts/components/prisma-schema/postgresql-extensions

[Jolg42]

We decided to only suggest extensions = [] for now.