node-api: factor out common code into macros

    * Create macro for checking new string arguments.
    * Create macro for combining env check and inside-gc check.

PR-URL: nodejs#50664
Reviewed-By: Vladimir Morozov <vmorozov@microsoft.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Chengzhong Wu <legendecas@gmail.com>