node:worker_threads did not implement isInternalThread, so both the
named import and the property on the default/require object were
missing:

import { isInternalThread } from "node:worker_threads";
// error: Uncaught SyntaxError: The requested module
// 'node:worker_threads' does not provide an export named
// 'isInternalThread'

isInternalThread was added in Node v23.7.0 / v22.14.0. It is a boolean
indicating whether the code runs inside an internal Node worker thread
(such as a module loader thread), as distinct from isMainThread. It is
false in the main thread and in user-created workers.

In the polyfill the synthetic ESM export names are derived from the keys
of the exports object, so adding isInternalThread there exposes it both
as a named export and as a property of the default/require object.
Deno has no internal Node worker threads, so the value is a constant
false.

Closes #35149