feat: change weekend days, fixes #283

[ghiscoding]

setting to allow changing weekend days, for example Friday/Saturday instead of Saturday/Sunday as per Discussion #283

However, it's currently broken when enabling iso8601: true, that would have to be fixed but I don't have more time to look into this. There's also probably a lot more code that calculate days/weekends somewhere else, so it's quite possible that I didn't cover everything. I assume that the iso8601 could probably be ignored but we would probably also need another setting to know which day is the starting (for example start on Sunday or Monday or even Tuesday)