node-api: include experimental feature flags in add-on version reported at runtime

This only applies to features that change the behaviour of existing APIs.

We currently have two flags for each experimental behaviour-changing API:

1. A feature flag indicating that it is available within the NAPI_EXPERIMENTAL portion of the API.
2. An opt-out flag indicating that, although the add-on chose to use NAPI_EXPERIMENTAL it does not wish to use this particular feature of the experimental bundle.

In order to avoid going down the experimental code path when running an add-on that chose to opt out of an experimental feature, we need to encode that choice in the version reported to the add-on loader.

If NAPI_EXPERIMENTAL is defined, all experimental features are turned on, unless, for any given feature, its opt-out flag is given.

We can include experimental flags in the version number reported at runtime as follows:

This value ends up on napi_env. At runtime we can check against the bits of this value to decide on the code path. We should probably create a family of macros for making decisions based on the feature flags.

At our Node-API meeting we concluded that we should introduce behaviour-changing features extremely sparingly, because, in order to ensure we do not break our users, even those that have opted into NAPI_EXPERIMENTAL, the testing complexity doubles with each feature we introduce.

実装ファイルやテストは指定されていません。まず、Node-API のバージョンが napi_env に到達する経路と、実験的機能が実行時にどのように表現されるかを追跡します。報告されるバージョンにオプトアウトの選択をエンコードし、リリース済みバージョンの動作を変更せずに、提案された feature flag のチェックを提供できれば完了です。