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: https://github.com/nodejs/node/pull/22259


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Kyle Farnung <kfarnung@microsoft.com>
Reviewed-By: Michael Dawson <michael_dawson@ca.ibm.com>