Support Node-API modules declaring their "Node-API version"

[kraenhansen]

Node-API modules can declare a version that they want to be initialized with, either:

• By setting nm_version in the napi_module passed to napi_module_register (depends on Support module registration via calls to napi_module_register #3).
• By exporting a node_api_module_get_api_version_v1 function (of type node_api_addon_get_api_version_func).

As per https://nodejs.org/docs/latest/api/n-api.html#node-api-version-matrix I believe this should default to 8 if not explicitly provided by the addon.