workers: replace message types string by constants

This change can prevent typos and redundant strings in code.

PR-URL: https://github.com/nodejs/node/pull/21537


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>