Fully type "hander()" in the api routes

[johannesschaffer]

The parameters (req & res) of the api routes in the "pages" dir (which are autogenerated by next-payload) don't have types. This makes it hard to use with Typescript strict mode (which Next.js comes with by default).

[JarrodMFlesch]

The code is now imported from the package, this way code changes can happen inside those handlers when the plugin is updated which is nice.

You will need to run yarn next-payload install and read through the updated readme!