Add support for descriptions on executable definitions

[glen-84]

graphql/graphql-spec#892 -> graphql/graphql-spec#1170

"Some description"
query SomeOperation(
  "ID you should provide"
  $id: String

  "Switch for experiment ...."
  $enableBaz: Boolean = false,
) {
  foo(id: $id) {
    bar
    baz @include(if: $enableBaz) {
      ...BazInfo
    }
  }
}

"Some description here"
fragment BazInfo on Baz {
  # ...
}