node-api: define version 9
Node-API version 9 promotes the following Node-API into stable:
node_api_throw_syntax_error
node_api_create_syntax_error
node_api_symbol_for
node_api_get_module_file_name
Runtime behavior changes introduced in https://github.com/nodejs/node/pull/47975 and https://github.com/nodejs/node/pull/47986 are still left experimental since they landed just in the past month and not backported to all LTS lines.
/cc @nodejs/node-api-implementer