Bug 1247687 - Implement InitModuleLoader methods for WorkerScriptLoader and WorkerGlobalScope; r=jonco

This implements a method to initialize the moduleLoader for workers. This will initialize only once, for all worker types (module and classic).

Depends on D147324

Differential Revision: https://phabricator.services.mozilla.com/D147326

js/loader/ModuleLoaderBase.cpp

@@ -982,7 +982,9 @@ void ModuleLoaderBase::Shutdown() {
   MOZ_ASSERT(mFetchingModules.IsEmpty());
 
   for (const auto& entry : mFetchedModules) {
-    entry.GetData()->Shutdown();
+    if (entry.GetData()) {
+      entry.GetData()->Shutdown();
+    }
   }
 
   mFetchedModules.Clear();