a way to get the root tag without having the user define it

RootTags are defined by the registry and then being used to init the native DOM.

This can be removed and reduced by handling the registry task in the library as well but the blocking factor is that
creating a root fiber node requires the global defs from the react library but then that's not exposed or exported
and so if we do use the legacy library to define the root container then we might get version locked to react-native's current version

either way, need to see how feasible it's going to be

Start by tracing how RootTags are defined by the registry and used to initialize the native DOM, then examine the root fiber node's dependency on the React library's global definitions. Define what a feasible solution would need to expose or replace, and consider whether it avoids locking the renderer to React Native's current version.

Written by the indexing model from the issue text.