events: re-use the same isTrusted getter

Creating a new function each time the property descriptor is set
comes with performance overhead, since these functions have different
identities, even if they contain the same code.

Refs: https://twitter.com/tverwaes/status/1285496612618473472

PR-URL: https://github.com/nodejs/node/pull/34459


Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Yongsheng Zhang <zyszys98@gmail.com>