Function isLeapYear

isLeapYear(year: number): boolean
Check if a year is a leap year

Leap year rules (Gregorian calendar):
- Divisible by 4: leap year
- EXCEPT divisible by 100: not a leap year
- EXCEPT divisible by 400: leap year
Examples: 2000 (leap), 1900 (not leap), 2024 (leap), 2100 (not leap)
Parameters
- year: number
  Full year number (can be negative for BCE)
Returns boolean
true if leap year, false otherwise
Example
```
isLeapYear(2000)  // true
isLeapYear(1900)  // false
isLeapYear(2024)  // true
```
- Defined in time/time-validation.ts:29