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.
Now that the lambda capture plugin isn't full of false-positives, we can
make the jump and start halting builds for these errors. It also allows
these plugins to be useful in CI.
