Add support for preserving dry-struct inheritance

[demsullivan]

I'd love if the dry-typescript compiler could identify dry-structs that inherit from one another, and preserve this when exporting TypeScript interfaces.

For example:

class A < Dry::Struct
  attribute :a, Types::String
end

class B < A
  attribute :b, Types::Integer
end

Should compile to:

export interface A {
  a: string;
}

export interface B extends A {
  b: number;
}