Option to not use TypeScript Namespaces

[pattrickrice]

Describe the solution you'd like
It seems like enums get generated in namespaces. eg.

export namespace Pets {
    export enum pet_type {
       Cat = "Cat",
       Dog = "Dog",
    }
}

Namespaces are unnecessary at this point; in addition the TypeScript team has gone through efforts for performance improvements in their own code base by removing namespaces: microsoft/TypeScript#51387.

It would be nice to have an option to not generate namespaces.

[gergan]

You could use --useUnionTypes, in this case the enums should be internalized