refactor: add compatibility for typst_syntax::LinkedNode.leaf_at

[ParaN3xus]

Add compatibility for typst_syntax::LinkedNode.leaf_at

[Enter-tainer]

LGTM

[Myriad-Dreamin]

Please don't use macro but use function instead whenever possible, because macros are not safely typed. For other code, also LGTM.

[Myriad-Dreamin]

Note we can apply some tricks to do better than provide functions. we can implement a LinkedNodeExt trait.

trait LinkedNodeExt {
  fn leaf_at_compat(&self, ...);
}

and usage:

use LinkedNodeExt;
let node = root.leaf_at_compat(pos + 1).unwrap();