events: convert errorMonitor to a normal property

Convert property errorMonitor to a normal property as non-writable
caused unwanted side effects.

Refs: https://github.com/nodejs/node/pull/30932#discussion_r379679982

PR-URL: https://github.com/nodejs/node/pull/31848


Reviewed-By: Michaël Zasso <targos@protonmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Matheus Marchini <mat@mmarchini.me>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>