Allow accessing path anchors inside a group

[wdeweijer]

It currently seems impossible to access anything other than named anchors inside a group.

#cetz.canvas({
  import cetz.draw: *

  group(name: "gruppe", {
    line(name: "lynne", (0,0), (2,3))
  })

  content("gruppe.lynne.start", [start]) // Does work
  content((name: "gruppe.lynne", anchor: 20%), [20%]) // Does not
  content((name: "gruppe", anchor: (name: "lynne", anchor: 70%)), [70%]) // Does not
})`

[johannes-wolf]

Same as #738.