Finalizer/NativeFinalizer token -> peer ?

context: https://github.com/dart-lang/language/blob/master/accepted/future-releases/1847%20-%20FinalizationRegistry/proposal.md

We have given the name "finalization token" or `token` to the value passed to the callback.

"token" is not a very descriptive name, we also call the `detachKey` a "key", but at least it has "detach" in the name. (And [sometimes](https://github.com/dart-lang/sdk/search?q=detachToken) we even call it `detachToken`!)

Should we rename `token` and "finalization token" to `peer` and "peer object" respectively?

cc @lrhn @mraleph

suggested by @mkustermann on https://dart-review.googlesource.com/c/sdk/+/229544/27..31/sdk/lib/_internal/vm/lib/finalizer_patch.dart#b40.