Add `mark:` for path decorations

[johannes-wolf]

Add mark support for coil, wave, zigzag.

[janosh]

just to write the code in a way that will render correctly in the future, will the mark: (end: "stealth") go on the line or the decoration?

#import "@preview/cetz:0.3.2": canvas, draw, decorations

#canvas({
  decorations.zigzag(
    draw.line(
      (-re-range, 0),
      (re-range, 0),
      mark: (end: "stealth"),
    ),
    amplitude: 0.1,
    segment-length: 0.2,
    name: "branch-cut",
  )
})

[johannes-wolf]

just to write the code in a way that will render correctly in the future, will the mark: (end: "stealth") go on the line or the decoration?

I think for decorations like zigzag, ... it should go on the decoration/modified path. Idea is to have different kinds of decorations. Some that alter the original path and some that add new elements without changing the original path. I am not yet sure, though.

[johannes-wolf]

There is a poc PR here: #541 but it needs to be re-done.
Marks could be decorations, too.