support prisma extensions

### What do you want and why?
handle `$extends()` prisma function (currently errors to `TypeError: (intermediate value).$extends is not a function`
- The types of the __enhanced__ `PrismaClient` break when extensions are added.


#### (Correct) DB types using `new PrismaClient()` without blitz enhancement (cannot be used due to [this](https://github.com/blitz-js/blitz/issues/4251#issuecomment-1909851287)):

### Additional Context
The error of using
```ts
const db = new EnhancedPrisma().$extends(...)
```

Start by reproducing the reported `TypeError` with `new EnhancedPrisma().$extends(...)` and compare its types with a plain `new PrismaClient()`. Trace how Blitz enhances Prisma and verify that extensions work at runtime and that the enhanced PrismaClient types remain usable.

Written by the indexing model from the issue text.