The upcoming GC functionality will require `WeakRef` and
`FinalizationRegistry`. Node.js v14 provides these as globals, but v12 does
not (there might be a command-line flag to enable it, but I think it's marked
as experimental). Rather than require all users upgrade to v14, we elect to
disable GC when running on v12.

This adds a local `weakref.js` module which attempts to pull `WeakRef` and
`FinalizationRegistry` from the global, and exports either the real
constructors or no-op stubs.

refs #1872
refs #1925

These two authorities are not part of SES, so they must be pulled from the
globals of the Start Compartment and ferried through the kernel to the vat
manager factory that calls makeLiveSlots.

This gives the outer layer of the vat (liveslots) access to nondeterminism.
We rely upon liveslots to not share this power with the user-level ocap-style
"vat code". Liveslots must never allow user-level code to observe behavior
that depends upon GC activity, because that activity is not part of the
specified input to the vat.

refs #1872

* fix: WeakRef taming follows taming pattern

* fix: error message

* fix: minor