worker: move worker thread setup code into the main script

This patch directly inlines `createMessageHandler()` and
`createWorkerFatalExeception()` in the new
`lib/internal/main/worker_thread.js` since the implementation
of the two methods are related to the execution flow of
workers.

PR-URL: https://github.com/nodejs/node/pull/25667


Reviewed-By: Anna Henningsen <anna@addaleax.net>