Firefox 146 adds non-registered symbols as WeakMap/WeakSet keys

[hamishwillee]

FF146 adds support for using non-registered symbols as keys for WeakMap and values for WeakSet in https://bugzilla.mozilla.org/show_bug.cgi?id=1966745

The issue only talks about WeakMap, but I ran a test for WeakSet and that behaves the same way:

const ws = new WeakSet();
const sym = Symbol('foo');
ws.add(sym); 

This updates the subfeature.

Related docs work can be tracked in mdn/content#41868

[github-actions]

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