perf(turbo-tasks-backend): use DefaultStorage for AggregationNumber to save memory (#88336)
This drops the size of the `aggregation_number` field from 16 to 12 bytes which allows the full `InnerStorage` struct to do from 136 bytes to 128 bytes (thanks alignment!)
Due to allocator behavior this is probably saving more like 32 bytes per InnerStorage allocation, see [mimalloc size classes](https://github.com/microsoft/mimalloc/blob/f0cd5505aa102cee991be0367b82506638a16281/src/init.c#L60)