feat: support for invisible nodes / distinct directional bias

[kokic]

This support is intended to allow the display of specified subtrees in their designated positions when drawing binary trees. For the current implementation, a simple approach is to make the branches connecting to a node disappear if the node's content is empty (exactly, space). Ideally, it should be adjusted to a uniquely designed type with a length.

For example:

let is_space = child.has("body") and child.at("body").has("amount")
if not is_space {
  place(line(stroke: stroke, start: (0pt+ child_width/2, lo), end: (children_width / 2 - x0+ child_width/2, hi)))
}

In this way, when we write #syntree("[A $quad$ B]"), we will obtain a binary tree with a distinct directional bias (node "A" nil (node "B" nil nil).