n-api: clean up thread-safe function

* Move class `TsFn` to name space `v8impl` and rename it to
  `ThreadSafeFunction`
* Remove `NAPI_EXTERN` from API declarations, because it's only needed
  in the header file.

PR-URL: nodejs#22259
Reviewed-By: Anna Henningsen <[email protected]>
Reviewed-By: Kyle Farnung <[email protected]>
Reviewed-By: Michael Dawson <[email protected]>