Make compatible with freezed package

[mkranus]

Freezed package requires the JsonKey annotation to be inserted into the constructor, which is then copied to the generated classes. But currently this use generates a warning, and this PR adds an additional TargetKind so the warning is no longer generated.

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[mkranus]

@googlebot I signed it!

[vi-k]

@JsonSerializable factory ...

-> The annotation 'JsonSerializable' can only be used on classes.

No TargetKind for factory.

[vi-k]

See https://github.com/rrousselGit/freezed/issues/474 and https://github.com/dart-lang/sdk/issues/46844

[princeteck]

Any update on this one @mkranus