functionality for creating an Appendix

[CarloLucibello]

Hi, thanks for the nice package, I started to use it to create some lecture notes.

Right now in order to create the appendix for the notes I do the following after the main text

#counter(heading).update(0)

#set heading(numbering: (..nums) => {
  if nums.pos().len() == 1 {
    return ""
  } 
  let a = nums.pos().slice(1)
  return numbering("A.1.1", ..a)
})

#align(center)[= Appendix]

== Gaussian Integrals  and Error Functions

== TODO

with the result

It took me some digging to reach this result, while I think it should be something much more immediate, as it is in latex.

Would you maybe consider adding some functionality in ilm to reach a similar result?