src: add NAPI_VERSION_EXPERIMENTAL

Refs: https://github.com/nodejs/node-addon-api/issues/421

Define NAPI_VERSION_EXPERIMENTAL so that it can be
used to guard code in addons that need to check
if a function they want to use is available.

PR-URL: https://github.com/nodejs/node/pull/25319


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>