Adding vectorial functionalities and examples

Firstly, from what I've seen so far, this package is quite impressive! It's nice to see that optics packages for Julia are being developed.

I think it would be useful and not too hard, as you remarked in the README, to implement the angular spectrum method for vectorial fields, as it is used quite often for structured light applications and nonparaxial optics. If you're interested, I can create a PR that allows users to do this. We can also discuss simple examples that illustrate the method.

Start by reading the README, especially its discussion of the angular spectrum method, then inspect the package's existing propagation functionality. Define the vectorial-field API and agree on simple examples with maintainers; done means the method is implemented and documented with working examples.

Written by the indexing model from the issue text.