Throw an error when a JSBI instance is implicitly converted via valueof.

[12wrigja]

While reviewing https://github.com/js-temporal/temporal-polyfill/pull/77 reviewers noticed several places in code where JSBI instances were implicitly being converted to NaN, which seems undesirable.

[12wrigja]

Open question: this does seem like a breaking change, so it warrants a major version bump? Any issues with that?

[jakobkummerow]

I suppose it should get a major version bump, yes. (Which is a bit silly for something so minor; but it does turn a potential silent failure into an exception, so I agree that strictly speaking that's a breaking change.)

[12wrigja]

Turns out that converting a JSBI instance into a native BigInt by passing it as the input to the constructor "just works" (globalThis.BigInt(JSBI.BigInt("10")) === 10n is true). This is because the constructor will fall back to calling toString if valueOf is not defined. By adding in a valueOf function that throws this no longer works - users would have to explicitly call toString on JSBI instances before passing that into the native BigInt constructor.

Are we ok with this sort of breaking change?