Integrate with @types/koa__cors

Hi

I want this project doesn't require additional typing library(which is @types/koa__cors) when coding in TypeScript. This could be solved by adding `.d.ts` file. Any ideas?

Start by reviewing the package entry point and the existing @types/koa__cors declarations, then determine where a .d.ts file would be included for TypeScript users. Done means the package provides the needed CORS typings without requiring the additional @types/koa__cors dependency.

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