feat(core): Add API to clear scope feature flags (#5426)

* feat(core): Add API to clear scope feature flags

Allow feature flags stored on a scope to be cleared without resetting other scope data. Scope.clear now also resets the feature flag buffer so stale flag evaluations do not carry over after clearing a scope.

Fixes #5422

Co-Authored-By: Claude <noreply@anthropic.com>

* changelog

---------

Co-authored-by: Claude <noreply@anthropic.com>