Common decorators

How about extending this project with some new decorators that would make use of the fact that both libraries are available?

For example: instead of using `@Type(() => Foo)` and `@IsInstance(Foo, {each: true})` and `@IsArray()` one could use a single decorator like `@IsArrayOf(Foo)` which would configure both class-transformer and class-validator with less code.

The issue names no files or tests. Start by reviewing the project's current decorator exports and how class-transformer and class-validator are combined, then define the scope for decorators such as IsArrayOf(Foo). Done should include a documented decorator API and coverage showing that transformation and validation both work together.

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