Support 'string' field type

Problem/Motivation

The 'string' field type is not supported by the widget (missing from field_types property in widget class annotation).

Steps to reproduce

Create the string field (named 'Text (Plain)' in UI).
Observe the Value widget may not be selected for it.

Proposed resolution

Add missing field type to the definition.

Remaining tasks

Instead of hardcoding the 'string' and other string-like field types (like URI), it could make sense to define supported field types dynamically with hook_field_widget_info_alter for all field widgets whose classes inherits \Drupal\Core\Field\Plugin\Field\FieldType\StringItemBase.

User interface changes

The Value widget would be selectable for Text (Plain) fields.

API changes

None.

Data model changes

N/A.