flutter-freezed plugin is incompatible Freezed (v3). (Minor change required)

**Is your feature request related to a problem? Please describe.**

The upgrade to the Freezed package from v2 to v3 comes with a breaking change in the syntax. Specifically, "Classes using the factory constructor now require a keyword sealed / abstract." [(details here)](https://github.com/rrousselGit/freezed/blob/master/packages/freezed/migration_guide.md)

**Describe the solution you'd like**

- Create a new major version of the flutter-freezed plugin that automatically includes `sealed`/`abstract` keyword.
- ~~Create a config flag that allows the user to specify if they want to add `sealed`/`abstract` before the class definition based on the version of Freezed they are using~~.

**Describe alternatives you've considered**

I've done a global replace on the file generated by the plugin add 'abstract' before the 'class' keyword.

So that this:

```typescript

@freezed
class Report with _$Report {
const Report._();

const factory Report({
required final String name
}) = _Report;

factory Report.fromJson(Map json) =>
_$ReportFromJson(json);
}

//...

```

Becomes:

```typescript
@freezed
abstract class Report with _$Report {
const Report._();

const factory Report({
required final String name
}) = _Report;

factory Report.fromJson(Map json) =>
_$ReportFromJson(json);
}

//...

```

...and that works as expected.

**Additional context**

The change to Freezed is described [here](https://github.com/rrousselGit/freezed/blob/master/packages/freezed/migration_guide.md).

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To: @Parables