support output to a directory, each endpoint to an seperate file

[zoubingwu]

I would also like to add a way to let end user to write a function to tell how to generate data for each endpoint.

may be something like:

defineMockData('endpointOperationId', () => {
  return {
    fieldA: 'random1',
  }
})

and the result will be shallow merged.

[zoubingwu]

close this since:

• now it always output to a folder
• let user hand write the logic about generating data is bad UX, we can now use ai for that
• there is no need for each handler to be a single file