src: introduce node::Realm

To distinguish per-context values from the node::Environment, split
those values to a new node::Realm structure and consolidate
bootstrapping methods with it.

PR-URL: nodejs#44179
Refs: nodejs#42528
Reviewed-By: Joyee Cheung <joyeec9h3@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>

test/pummel/test-heapdump-env.js

@@ -38,4 +38,10 @@ validateSnapshotNodes('Node / Realm', [{
   ]
 }]);
 
+validateSnapshotNodes('Node / Realm', [{
+  children: [
+    { node_name: 'process', edge_name: 'process_object' },
+  ]
+}]);
+
 console.log(context);  // Make sure it's not GC'ed