d.ts: Export PendingInterceptor type from mock-agent.d.ts

[KristjanTammekivi]

Allow the type to be addressable

Currently the type can't be addressed, so You can't do

const checkPendingInterceptors = (interceptors: PendingInterceptors) => {
  //
};

outside of the project

The implementation should look like...

I would move the interface declaration under the MockAgent namespace and change the internal usages to

  pendingInterceptors (): MockAgent.PendingInterceptor[]

I have also considered...

Copying the interface declaration to my project, but this could cause desync if the interface is ever changed in Undici

Additional context