Add locale for en-SG

[compleatguru]

Hi!
Greetings from Singapore.
Will it be possible to have Singapore data?

A possible data source is https://www.mom.gov.sg/employment-practices/public-holidays that gives iCal file.

[lauft]

We can add a locale for Singapore, but we need more information:

• Please provide a list of all holidays, i.e. days "which are defined by law on which business or work are suspended or reduced"
• For each holiday
  • What is the official name?
  • Is it a regional or nation-wide holiday?
  • If regional, what are the regions the holiday is observed in? (Note: currently only regions defined in ISO_3166-2 are supported)
  • Is it a fixed or variable date?
  • If fixed, provide the date in format MM-DD
  • If a holiday has a variable date: What is the algorithm to calculate it (e.g. nth / in ? If it depends on the easter date: Is it the western or orthodox easter date?
  • If a holiday is proclaimed by the government: Can you provide the source?
• Are holidays moved to a different date, e.g. if they fall on a saturday/sunday/...? If yes, which holidays and what is the algorithm?
• Are holidays observed on a different date (is there a substitute holiday), e.g. if they fall on a saturday/sunday/...? If yes, which holidays and what is the algorithm?
• Have there been any holidays renamed/deleted/created since 2011? If yes, which ones and since when?
• Can you provide the legal sources which define the holidays?

[lauft]

Implemented with #143