An action that reaches the host (language.md, section 6.3):
continue_conversation from @ToAssistant, open_url from @OpenUrl, or
the name of a library action.
form_state is read when the action runs. Inside a form it holds that
form's fields, each as %{"value", "componentType"}; outside one it holds
the page's $variables and page-level fields.
Summary
Functions
The wire form of language.md, section 6.3.