Feature request: Function to calculate solar terminator

[pepijn-devries]

Thank you for your work on the suncalc package.

It would be great if I could use this package to calculate the solar terminator (i.e., the line across the earth that separates day from night at a given date and time). For now I'm using base::optim in combination with the suncalc functions to solve the problem numerically, But I bet there is a more efficient way to do this.

Hope you are willing to consider adding this feature to the package...