Add new subdivision "DE-SH-INS" for Schleswig-Holstein Islands and added the specific dates for summer and autumn holiday (years 2010 - 2030)

[SEPAHL]

As stated here: https://www.schleswig-holstein.de/DE/landesregierung/themen/bildung-hochschulen/ferientermine

On the islands of Sylt, Föhr, Amrum, and Helgoland as well as on the Halligen, the summer holidays end one week earlier; the autumn holidays begin one week earlier. In the 2029/30 school year, the first day of the autumn holidays on these islands is Friday, September 28, 2029, and the last day is Friday, October 19, 2029.

To implement this difference, a new subdivision "DE-SH-INS" (INS = abbreviation for German word for Islands "INSeln") has been added for Schleswig-Holstein Islands and the specific dates for summer and autumn holidays have been added for the years 2010 - 2030

[fstueber]

Will merge/overtake this later because I want to model this different (by introducing a concept of exceptions for holidays). And thank you very mich for your work!!!

[fstueber]

Please check refactored version of Schleswig-Holstein school holidays.

[fstueber]

The changes were made without merging. Therefore, I will close this PR. Thank you for your contribution.