Export FluentIcon type via a publicly importable export

[dlannoye]

The FluentIcon type used by createFluentIcon and bundleIcon in @fluentui/react-icons is currently not importable without resorting to a deep path not exposed by the exports property in package.json. This type is defined in ./utils/createFluentIcon, but is not exported via the index leading to overhead when trying to build our own custom icons.

Ideally this type would be exposed since it is needed for the types of other public APIs.