GC-allocated objects should never have JS::SafeFunction/JS::Handle
fields.
For now the plugin only emits warnings here, as there are many cases
of this occurring in the codebase that aren't trivial to fix. It is also
behind a CMake flag since it is a _very_ loud warning.
This is a more general and robust replacement of the LibJSGCVerifier.
We want to add more generic static analysis, and this new plugin will
be built in a way that integrates into the rest of the system.
