New rule: Report unused import expressions

[FloEdelmann]

I'd like to suggest a rule that reports # import expressions that are not used in the current file:

Fail

# import Foo from 'someOtherFile.graphql'
#        ^^^ Fragment 'Foo' not used in this file, import expression can be removed.

query MyQuery {
  bar
}

# import 'someOtherFile.graphql'

query MyQuery {
  fooField
}

Pass

# import Foo from 'someOtherFile.graphql'

query MyQuery {
  fooField {
    ...Foo
  }
}

# import 'someOtherFile.graphql' # assuming it defines a `Foo` fragment

query MyQuery {
  fooField {
    ...Foo
  }
}

[dimaMachina]

Accepted