fix: clamp day-of-month when adding years/months (calendar overflow)

[spokodev]

toSeconds/end overflow the calendar when the start day does not exist in the target month.

import { parse, toSeconds } from "iso8601-duration";

// Jan 31 2021 + P1M
toSeconds(parse("P1M"), new Date(2021, 0, 31)) / 86400; // 31  -> Mar 3
// expected 28 -> Feb 28

end adds years/months with the raw Date setters:

then.setMonth(then.getMonth() + duration.months);

Setting the month on Jan 31 rolls over to Mar 3 instead of clamping to Feb 28. The same happens for years (Feb 29 + P1Y becomes Mar 1 instead of Feb 28).

The @js-temporal/polyfill that the suite validates against constrains the day to the target month:

Temporal.PlainDate.from("2021-01-31").add({ months: 1 }).toString(); // 2021-02-28

Fix: add years/months on the 1st of the month, then clamp the day to the number of days in the resulting month before adding the day component. Multi-unit and leap-year cases already handled by Date (e.g. P2M from Jan 31 stays Mar 31) are unchanged.

Adds regression tests for the month, year, and month-then-day cases.