feat: Allow resizing split panes

[TeijiW]

Hey! First of all, loving Rio so far. One feature I'm really missing is the ability to resize splits after creating them.

Context and current situation

When I create splits, they're fixed in size and I can't adjust them. This becomes a bit frustrating when I need more space for one pane (like when checking logs or working with multiple files).

What would be nice to have

Being able to drag the split border with the mouse to resize
Maybe some keyboard shortcuts to adjust sizes (like Alt+Arrow keys)

Why this matters

I often work with multiple panes and need to give more space to certain tasks. Right now, the only way to get different sized splits is to close and recreate them, which breaks my workflow.

Let me know if you need any clarification or have questions! Happy to provide more details if needed.