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.
Checklist
-
make -j4 test
(UNIX), orvcbuild test
(Windows) passes -
commit message follows commit guidelines