+ holidays for Singapore

The holidays I have added are mostly gazetted holidays, and several informal days.

I didn't include some holidays that are celebrated in Singapore,  which are:

- Chinese New Year (requires the usage of a lunar calendar, plausible but tedious to do)
- Hari Raya Puasa and Hari Raya Haji (Islamic calendar)
- Vesak Day (falls on the first full moon in May)
- Deepavali (15th day of a specific month in the Hindu calendar)