chore: mark forceOptimisticNavigation as internal (#47905)
### What?
Hide internal API.
### Why?
Ref: https://vercel.slack.com/archives/C035J346QQL/p1669125601246609?thread_ts=1669125167.045259&cid=C035J346QQL
### How?
Mark the internal property with `@internal` so it's not shown by IntelliSense.
Related #47894
