'list_text' field type renamed to 'list_string'

The Drupal 7 'list_text' field type, that stores raw string values selected among a limited admin-defined set of allowed values, has been renamed 'list_string' in Drupal 8.

This is in order to stay consistent with the nomenclature of other core textual field types after they were split into separate field types for formatted and unformatted text:
- 'string*' field types are about raw (unformatted) strings
- 'text*' field types are about formatted text (string + format)

Note: earlier in Drupal 8 the class implementing the field type was \Drupal\options\Type\ListTextItem, it was been renamed to \Drupal\options\Type\ListStringItem.