Feature Request: Introduce the possibility to use `{{param}}` notation as alternative to single `{param}` notation

In many prompts the context is JSON data. By using single curly brackets notation for template parameters means that I need to escape all the `{` and `}` in the input JSON data.

I propose to introduce a flow setting to switch from single curly brackets to double curly brackets notation. By setting the property on the flow would guarantee backward compatibility with the current notation and would apply to all nodes (text, prompts) etc in the flow. Alternatively, the option could be enabled on each node separately (more flexible but could be source of errors) or as a global setting (simpler, but might generate errors or unexpected results if a flow with the other notation is loaded and executed).

The double curly brackets notation is used by the widely adopted [Mustache](https://mustache.github.io/)