Support for the `marks (array of string)` field in TREE reply

[brownjohnf]

Unless I'm missing something (which is quite possible!) it looks like the marks field of the IPC response to the get_tree method is not present in i3ipc-rs. Is there a reason it's not present? There's a get_marks method that returns the currently used marks, but there's no way to correlate them with the nodes that have the mark, AFAICT.

I'm open to making a PR to add this; I wanted to confirm I'm right and check in first.