Declare that TextTokenInterface and TextValueInterface implement \Stringable

Note that \Stringable was added in PHP 8.0, so referencing it will be an implicit dependency on PHP 8.0 or higher.

However, that should be fine, as the .info.yml files in the 8.x-1.x branch dropped support for Drupal 9 in 📌 Drop support for Drupal 9 Fixed (committed 2023-11-11), and Drupal core dropped support for PHP 7 starting with 9.4 → (i.e.: Drupal ^9.4, ^10, and ^11 all require at least PHP 8.0).

Tests pass, so I'm going to mark this as "Needs review".

Thanks a lot for creating the ticket and merge request. Looks all good to me.
Merging. Thanks again!

Automatically closed - issue fixed for 2 weeks with no activity.