Since we made the default for Node.js core finalizers synchronous for
users running with `NAPI_EXPERIMENTAL` and introduced
`env->CheckGCAccess()` in Node.js core, we must now defer all
finalizers in node-addon-api, because our users likely make non-gc-safe
Node-API calls from existing finalizers. To that end,
  * Use the NAPI_VERSION environment variable to detect whether
    `NAPI_EXPERIMENTAL` should be on, and add it to the defines if
    `NAPI_VERSION` is set to `NAPI_VERSION_EXPERIMENTAL`, i.e.
    2147483647.
  * When building with `NAPI_EXPERIMENTAL`,
    * render all finalizers asynchronous, and
    * expect `napi_cannot_run_js` instead of `napi_exception_pending`.