Make NavigationDestination and NavigationHistoryEntry constructible

[domenic]

@tbondwilkinson mentions that it's helpful to sometimes create synthetic versions of these objects for unit testing purposes. This would also align with https://w3ctag.github.io/design-principles/#constructors .

I think we would not do any significant validation on the inputs, e.g. we would not validate key/id at all besides casting the input to a string. (Possible validations would include checking that they're valid UUIDs, checking that they're valid v4 UUIDs, and checking that they're appropriate key/id UUIDs of actual history entries.)

The only potential downside of this is that unsuspecting code might end up getting synthetic NavigationDestination or NavigationHistoryEntry instances that break the invariants the code usually relies on. E.g. navigation.entries()[entry.index] === entry would always be false if entry is synthetic.