worker: set --experimental-worker always

This is similar to nodejs#25361 in
functionality, but allows avoiding some backporting pain for v11.x.

Refs: nodejs#25361

Author: addaleax

src/node_options.h

@@ -95,7 +95,7 @@ class EnvironmentOptions : public Options {
   bool experimental_modules = false;
   bool experimental_repl_await = false;
   bool experimental_vm_modules = false;
-  bool experimental_worker = false;
+  bool experimental_worker = true;
   bool expose_internals = false;
   std::string http_parser =
 #ifdef NODE_EXPERIMENTAL_HTTP_DEFAULT