[Generator] preliminary data class generation and @generic annotation

[astralstriker]

• Can specify the enum annotated with @sealed as @generic.
• Can specify field annotated with @Data() as @generic
• Cannot specify field annotated with @Object() as @generic
• Can add a list of DataFieldsto fields parameter of @Data()
• Iff a field annotated with @Data() also has @generic, it must have at least one Generic DataField (check example/lib/sealed_test.dart)
• Iff a field annotated with @Data() also has @generic, its parent enum must be annotated with @generic

[xsahil03x]

LGTM