Restore CaseStringHelper as @api

[TomasVotruba]

CaseStringHelper was removed in #492 along with YamlToAttributeDoctrineMappingRector, but it is still used downstream by rector-drupal.

Restored as-is and marked @api, so class-leak skips it despite having no in-repo usage:

/**
 * @api used by rector-drupal
 */
final class CaseStringHelper
{
    public static function camelCase(string $value): string
    // ...
}

Covered with a unit test that pins current behavior:

yield ['some_property', 'someProperty'];
yield ['someProperty', 'someProperty'];
yield ['some property', 'someProperty'];
yield ['some-property', 'someproperty'];