feat: support mouse dragging panel separators to resize panelsadded drag column function

[winlaic]

Implements tmux-style mouse dragging of the panel separators to dynamically resize the parent, current, and preview panels (see the gif below).

Changes

yazi-config/preset/yazi-default.toml

• Added "drag" to the default mouse_events list so drag handling is enabled by default.

yazi-actor/src/app/mouse.rs

• Added render!() after the Root:drag() Lua call so the UI repaints immediately on every drag event.
• Without this, dragging updated the layout state but the resize was not visible until some unrelated event triggered a redraw.

yazi-plugin/preset/components/root.lua

• Added a _drag_which field (nil | "left" | "right") to track which separator is currently being dragged.
• Extended Root:click() to detect mouse-down on either separator (with a ±1 column tolerance), set _drag_which, and clear it again on mouse-up.
• Implemented Root:drag() to read the current panel widths from tab._chunks, compute the new widths from the cursor position, and write the updated all-pixel-width array back to rt.mgr.ratio.

How it works

mouse down near separator  ->  Root:click()  sets _drag_which
mouse drag                 ->  Root:drag()   updates rt.mgr.ratio + render!()
mouse up                   ->  Root:click()  clears _drag_which

The separator hit zones are derived from tab._chunks, which contains the three column rects produced by Tab:layout()
Each separator uses a ±1 column tolerance and is only active when the corresponding side panel is visible (width > 0).