fix(neon): Tag Root with an instance id and verify before using

[kjvalencik]

Overview

Root are Neon's mechanism for holding persistent references to values on the JavaScript heap. They are both Send and Sync to allow holding references across Rust threads. However, the current implementation has a soundness hole.

Resolves #843

Problem

When using Node worker threads, there may be multiple JavaScript runtimes present in the process. As shown in #843, it is possible to use a Root with a Context from the wrong runtime and cause a crash/undefined behavior.

Solution

This PR resolves the issue by panicking when attempting to dereference from the wrong module instance. In the future, we may want to add try_ variants to the Root methods to allow error handling without a panic. However, in most cases, this will be a logic error that applications cannot recover.

Approach

On Node-API 6+, we use instance data to store a unique identifier initialized from a global counter. Each Root stores the identifier for comparing later when being used.

On previous versions, the ThreadId is used. This may not be as robust and is sensitive to V8 implementation details.

[dherman]

This is so straightforward and non-clever (in a good way). LGTM!