Function getFestivalDate

Traditional lunar calendar festivals with region-aware naming. Different regions may observe different festivals or use different names.

getFestivalDate(
    config: CalendarConfig,
    key: string,
    gyear: number,
): GregorianDate | null
Get the Gregorian date of a specific festival in a given year
Parameters
- config: CalendarConfig
  Calendar configuration
- key: string
  Festival key (e.g., 'spring', 'mid-autumn', 'new-year-eve')
- gyear: number
  Gregorian year
Returns GregorianDate | null
Gregorian date of the festival, or null if the key is not found
Example
```
getFestivalDate(CHINESE_CONFIG, 'spring', 2024)       // { year: 2024, month: 2, day: 10 }
getFestivalDate(CHINESE_CONFIG, 'mid-autumn', 2024)   // { year: 2024, month: 9, day: 17 }
getFestivalDate(CHINESE_CONFIG, 'new-year-eve', 2024) // { year: 2025, month: 1, day: 28 }
```
- Defined in festival.ts:234