worker: restrict supported extensions

Only allow `.js` and `.mjs` extensions to provide future-proofing
for file type detection.

Refs: https://github.com/ayojs/ayo/pull/117


Reviewed-By: Stephen Belanger <admin@stephenbelanger.com>
Reviewed-By: Olivia Hugger <olivia@fastmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>

PR-URL: https://github.com/nodejs/node/pull/20876


Reviewed-By: Gireesh Punathil <gpunathi@in.ibm.com>
Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>
Reviewed-By: Shingo Inoue <leko.noor@gmail.com>
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>
Reviewed-By: Tiancheng "Timothy" Gu <timothygu99@gmail.com>
Reviewed-By: John-David Dalton <john.david.dalton@gmail.com>
Reviewed-By: Gus Caplan <me@gus.host>