src: rename WorkerThreadMain to PlatformWorkerThread

Refs: https://github.com/nodejs/node/pull/21079#discussion_r194251183

PR-URL: https://github.com/nodejs/node/pull/21982


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>
Reviewed-By: Ali Ijaz Sheikh <ofrobots@google.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>