Retrieves processing information about a field from $form_state.
$parents: The array of #parents where the field lives in the form.
$field_name: The field name.
$langcode: The language in which the field values are entered.
$form_state: The form state.
An array with the following key/data pairs:
function field_form_get_state($parents, $field_name, $langcode, &$form_state) {
$form_state_parents = _field_form_state_parents($parents, $field_name, $langcode);
return NestedArray::getValue($form_state, $form_state_parents);
}