When we launch ReadingLists as a beta feature on web, we want new accounts created after a configurable date to have the beta feature enabled by default, while still allowing users to opt out via their preferences.
We should define a config e.g. `$wgReadingListsBetaDefaultForNewAccountsAfter` that is a timestamp e.g. '20260301000000' or null (if disabled)
Then, we can use `$wgConditionalUserOptions` setting in core with `CUDCOND_AFTER` to set the default value of the ReadingLists beta feature preference to enabled for accounts registered after a configurable timestamp.
To do this, we need to define a registration callback for Setup::onRegistration that sets `$wgConditionalUserOptions['readinglistsbeta']`.
See: https://www.mediawiki.org/wiki/Manual:$wgConditionalUserOptions
NOTE: Constants::PREF_KEY_BETA_FEATURES is a constant in HookHandler.php in ReadingLists, that can be used instead of hardcoding 'readinglistsbeta'