Add support for Dart primary constructors

[SidNewman]

Hi!

Context

Dart 3.13 introduced primary constructors, allowing constructor parameters and fields to be declared directly as part of the class declaration.

Quicktype currently generates Dart models using traditional field declarations and constructors.

For example:

class User {
  final int id;
  final String name;

  User({
    required this.id,
    required this.name,
  });
}

With Dart's primary constructor syntax, this can now be expressed more concisely:

class User({
  required final int id,
  required final String name,
});

Proposal

It would be useful for the Dart renderer to optionally generate models using primary constructors.

For example, a CLI option such as:

quicktype \
  --src person.json \
  --src-lang json \
  --lang dart \
  --top-level Person \
  --primary-constructors \
  --out person.dart

Could generate:

class User({
  required final int id,
  required final String name,
});

Instead of:

class User {
  final int id;
  final String name;

  User({
    required this.id,
    required this.name,
  });
}

Why make it optional?

Primary constructors require a recent Dart SDK, so changing the default output could break generated code for projects using older Dart versions.

Making this an opt-in renderer option would preserve backwards compatibility while allowing projects using modern Dart to take advantage of the newer syntax.