`is-gen-fn` is has invalid package.json in Node 16
The package `is-gen-fn`, which `koa-joi-router` has a dependency on, has an issue in its package.json resulting in a `DeprecationWarning` when `koa-joi-router` is used in node 16. 

>[DEP0128] DeprecationWarning: Invalid 'main' field in '/usr/src/app/node_modules/is-gen-fn/package.json' of 'yes'. Please either fix that or report it to the module author

As the `is-gen-fn` package has not been updated in 7 years nor is it public, and is a single line function I recommend that you implement the function in this package to avoid issues in the future.

Function from `is-gen-fn`

```
function isGeneratorFunction(obj) {
  return obj && obj.constructor && 'GeneratorFunction' == obj.constructor.name;
}
```