Up until version 9, Node-API versions were additive and versioned

independently from Node.js. This meant that any version was

an extension to the previous version in that it had all of

the APIs from the previous version with some additions. Each

Node.js version only supported a single Node-API version.

For example v18.15.0 supports only Node-API version 8. ABI stability was

achieved because 8 was a strict superset of all previous versions.

As of version 9, while Node-API versions continue to be versioned

independently an add-on that ran with Node-API version 9 may need

code updates to run with Node-API version 10. ABI stability

is maintained, however, because Node.js versions that support

Node-API versions higher than 8 will support all versions

between 8 and the highest version they support and will default

to providing the version 8 APIs unless an add-on opts into a

higher Node-API version. This approach provides the flexibility

of better optimizing existing Node-API functions while

maintaining ABI stability. Existing add-ons can continue to run without

recompilation using an earlier version of Node-API. If an add-on

needs functionality from a newer Node-API version, changes to existing

code and recompilation will be needed to use those new functions anyway.

In versions of Node.js that support Node-API version 9 and later, defining

`NAPI_VERSION=X` and using the existing add-on initialization macros

will bake in the requested Node-API version that will be used at runtime

into the add-on. If `NAPI_VERSION` is not set it will default to 8.