feat(client): Support `$extends` feature about prisma-dart

[medz]

@nestharus
I don't have much plan to implement it in Dart 3.x. Once $extends is implemented in Prisma Dart, it will become a counterexample that API consistency cannot be guaranteed.

The premise that $extends appears in Prisma Client is that TS/JS do not have a good language-level mix. Prisma does not want users to write complex code to achieve type-safe extension functions.

Prisma uses TS type generates to achieve this, but it will never be possible in Dart. Using it in Dart is a counterexample.

Maybe in the future, when there is a better solution after the macro is officially launched, there is no breakthrough at the language level, or the design is incompatible, Prisma Dart will shelve some Prisma features indefinitely.