Allow Outputting Anchors for Tree Nodes

[johannes-wolf]

Discussed in https://github.com/cetz-package/cetz/discussions/698

^{Originally posted by **noahjutz** September 23, 2024} Is it possible to refer to a tree's node's position from outside of a tree? When I try ``` cetz.tree.tree( // ... draw-node: (node, p) => { if node.content.text == "1" {circle((), radius: 0, name: "anchor_1")} else if node.content.text == "2" {circle((), radius: 0, name: "anchor_2")} } ) line("anchor_1", "anchor_2") ``` I get `Assertion failed: Unknown element 'anchor_1' in elements ()`. Motivation: I want to draw arrows between sibling nodes in a tree.

[johannes-wolf]

Forward anchors from the draw-node function to the tree group.