Add EventDispatcher and tab events API

Introduce a new EventDispatcher and three API events (TabDisplayNameEvent, TabHeaderFooterEvent, TabTeamUpdateEvent) to allow plugins to transform tab list components. Events are fired with a 50ms timeout and snapshot-based rollback to avoid partial mutations from slow or failing listeners. Wire the dispatcher into the Velocitab API initialization and use it to: transform team packet components in ScoreboardManager (CREATE/UPDATE), transform header/footer in TabPlayer, and transform per-viewer display names in PlayerTabList.