layout method
Computes and returns pane layout for a given terminal size.
Implementation
PaneLayout layout({required int width, required int height}) {
final panes = <String, PaneRect>{};
final splits = <String, SplitHandle>{};
_layoutNode(root, 0, 0, width, height, panes: panes, splits: splits);
return PaneLayout(panes: panes, splits: splits);
}