fix(isRFC3339): reject impossible calendar dates

[spokodev]

isRFC3339 is a pure regex test, so it returns true for dates that cannot exist:

validator.isRFC3339('2021-02-30T00:00:00Z'); // true  (Feb has 28/29 days)
validator.isRFC3339('2021-04-31T00:00:00Z'); // true  (Apr has 30 days)
validator.isRFC3339('2021-06-31T12:00:00Z'); // true  (Jun has 30 days)
validator.isRFC3339('2021-02-29T00:00:00Z'); // true  (2021 is not a leap year)

RFC 3339 section 5.6 restricts date-mday to the number of days in the given month and year, which the regex cannot express ([12]\d|0[1-9]|3[01] allows 01-31 for every month). The existing test block already treats impossible dates as invalid (2009-13-19, month 00, day 00); this extends the same intent to the day-of-month maximum and the leap-year rule. For contrast, isISO8601(..., { strict: true }) already rejects 2021-02-30.

Fix

After the regex passes, compare the day against a per-month maximum, with a leap-year check for February:

function daysInMonth(year, month) {
  if (month === 2) {
    return ((year % 4 === 0 && year % 100 !== 0) || year % 400 === 0) ? 29 : 28;
  }
  return [31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31][month - 1];
}

The comparison is arithmetic rather than new Date(...), which avoids the legacy two-digit-year behavior of the Date parser for RFC 3339's valid years below 100. The seconds field is never inspected, so the leap-second value 14:53:60Z (already in the valid list) stays valid.

Tests

Extended the RFC 3339 block in test/validators.test.js:

• valid: 2020-02-29 and 2000-02-29 (leap day, and the div-by-400 century case)
• invalid: 2021-02-30, 2021-04-31, 2021-06-31, 2021-02-29, and 1900-02-29 (the div-by-100 non-leap century case)

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (d4e02ee) to head (30fb217).

Additional details and impacted files

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##            master     #2841   +/-   ##
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  Coverage   100.00%   100.00%           
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  Files          114       114           
  Lines         2598      2605    +7     
  Branches       658       661    +3     
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+ Hits          2598      2605    +7     

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