node-api: include experimental feature flags in add-on version reported at runtime about node HOT 3 OPEN

@nodejs/node-api

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Hi @gabrielschulhof ,

Just wanted a clarification with an example. Say we add a new Node-API, node_api_x(). Since this is not a behavior change, we would continue with our current flow (ie. guard declaration via #ifdef NAPI_EXPERIMENTAL, then update to #if NAPI_VERSION >= x on stable release, etc etc following https://github.com/nodejs/node/blob/main/doc/contributing/adding-new-napi-api.md), correct?

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@KevinEady that's a good question. I think we would still add a feature flag, so we'd know the feature was available, though not an opt-out flag. I think assembling the version to report would be encoded using a series of macro calls starting with NAPI_VERSION and adding each successive feature bit. Thus we could, for non-behaviour-changing features, avoid updating this series of macro calls with a call to setting the feature bit for the non-behaviour-changing feature. That way, a feature bit would not be reserved for the feature, since it's not needed at runtime anyway.

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