Add TypeScript types for handlers

**Feature request**
It would be great to have notifications for models.

**Use case**
Many users need this functionality for production deployments.

**Implementation idea**
Add a new config option `enable_notifications` with default `false`.

Start by locating the handler type definitions referenced by the title and the configuration entry for `enable_notifications`. Compare the requested TypeScript types with the notification use case and determine how model notifications fit the existing configuration; done means the scope is resolved and the option's expected behavior is specified.

Written by the indexing model from the issue text.