Function isGregLeapYear

isGregLeapYear(year: number): boolean
Returns true if the Gregorian year is a leap year
Parameters
- year: number
  Gregorian year
Returns boolean
Example
```
isGregLeapYear(2000); // true
isGregLeapYear(2020); // true
isGregLeapYear(2023); // false
isGregLeapYear(2100); // false (divisible by 100 but not 400)
```
- Defined in greg.ts:55