feat: add Split Button

- The goal is to implement the new Split Button component
- Reuse common logic from https://github.com/callstack/react-native-paper/issues/4928 (PR already open)
- Reuse newly introduced tokens (shape, motion, typography, color etc)
- Extract component specific tokens for easier maintenance.
- Get inspired from TextInput/TextField, Switch, Checkbox, FAB components which have gone through modernization.
- Showcase the component in the example app
- Update the documentation

Start by reviewing the common logic in issue 4928 and the modernization patterns in TextInput/TextField, Switch, Checkbox, and FAB. Trace how the example app showcases components and how documentation is organized. Done means a Split Button using the shared and component-specific tokens, demonstrated in the example app, with documentation updated.

Written by the indexing model from the issue text.