Show the Hot Reload button for Dart Web apps when supported

We show the hot reload button for flutter apps (determined by the presence of the `reloadSources` service extension and called by calling the service extension) and for Dart CLI apps (expected to be present and called by calling `VmService.reloadSources` directly).

For Dart web apps, we need to figure out how we can detect whether hot reload is supported and then we need to know how we can call it.