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
