feat(ecmascript): FinalizationRegistry

[aapoalas]

This implements half of #623. I also tried out using SoAVec as the storage for FinalizationRegistryRecords and it works pretty wonderfully from a code-organisation point of view at least. Methods that have to do with registration belong on the Cells struct and touch only one cache line, methods that have to do with cleanup belong on the Cleanup struct and touch only one cache line, the backing object is a third cache line, and finally the only time when these cache lines interact is during GC. Nice.