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.

Es werden keine Implementierungsdateien oder Tests genannt. Beginne damit nachzuverfolgen, wie die Node-API-Version napi_env erreicht und wie experimentelle Funktionen zur Laufzeit dargestellt werden; als erledigt gilt die Aufgabe, wenn Opt-out-Entscheidungen in der gemeldeten Version codiert sind und die vorgeschlagenen Feature-Flag-Prüfungen bereitgestellt werden, ohne das Verhalten veröffentlichter Versionen zu ändern.