Remove js.ExecutionWorld

The ExecutionWorld doesn't do anything meaningful. It doesn't map to, or
abstract any, v8 concepts. It creates a js.Context, destroys the context and
points to the context. Those all all things the Env can do (and it isn't like
the Env is over-burdened as-is).

Plus the benefit of going through the Env is that we can track/collect all
known Contexts for an isolate in 1 place (the Env), which can facilitate things
like context creation/deletion notifications.

src/browser/Browser.zig

@@ -53,7 +53,7 @@ notification: *Notification,
 pub fn init(app: *App) !Browser {
     const allocator = app.allocator;
 
-    var env = try js.Env.init(allocator, &app.platform, &app.snapshot);
+    var env = try js.Env.init(app);
     errdefer env.deinit();
 
     const notification = try Notification.init(allocator, app.notification);