Fix leaking Ref in weakMapMemoize

[tomavos]

This PR contains the following changes:

• Update the dereference logic in weakMapMemoize.ts to check if the lastResult is a Ref before dereferencing it.

Context:

The previous code could leak a WeakRef if the dereferenced value was undefined (which I think happens when the original object is forcefully garbage collected): Let lastResult be a Ref to undefined:

const lastResultValue = lastResult?.deref?.() ?? lastResult
-> const lastResultValue = lastResult.deref() ?? lastResult
-> const lastResultValue = undefined ?? lastResult`
-> const lastResultValue = lastResult (= Ref{undefined})

With this new change, we will return the new value (even when the old garbage collected value would've passed the resultEqualityCheck and would've been memoized).

I chose the instanceof check because Ref is defined above as a WeakRef if it is available, else it is a StrongRef which is a locally defined class.

I tried to add a unit test which reproduced the issue by garbage collecting values in the cache so they would be Refs to undefined, but I didn't have any luck. I followed a similar strategy to this test by using a FinalizationRegistry. I also tried a test which unmounted a React component containing the original state. Please let me know if you have any ideas for this unit test :)

(aside) How we discovered the issue:

• We had an array in a React component, which was then fed into a chain of selectors (of which the last returned a weakMapMemoized array).
• This component got unmounted, and the cached weakMapMemoized result got garbage collected.
• The next caller of this same selector produced an empty array result.
• This passed our shallowEquals resultEqualityCheck (Object.keys(WeakRef{undefined}) == Object.keys([]) == [])
• This leaked the WeakRef into the code which expected an array, and caused an exception.

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