polyfill: fix Instant precision bug
polyfill: fix Instant precision bug

Fix a bug where Instant.prototype.toString produced the incorrect result
where the fractional part included leading zeroes and precision was not
auto or 0. Includes test262 test for the same.

Fixes: https://github.com/tc39/proposal-temporal/issues/1336

js-temporal/temporal-polyfill,js-temporal/temporal-polyfill,js-temporal/temporal-polyfill