Messages are consumed by useShinyMessageHandler(id, handler) on the
React side. The id is namespaced to the current Shiny module (if any)
so module-scoped handlers match, just like input/output ids.
Usage
send_message(session, id, data)
Arguments
session
The Shiny session.
id
Message id; must match the id passed to
useShinyMessageHandler() in the React component.
