Rename Durable Object script_name to service?

[GregBrimble]

This would make it consistent with the service bindings.

[durable_objects]
  bindings = [
    { name = "TEST_OBJECT", class_name = "", script_name = "", environment = "" }
  ]

services = [
  { binding = "TEST_BINDING", service = "", environment = "" }
]

[penalosa]

cc @admah