feat: expose pointerType in ResizeEvent

[tomkp]

Summary

Exposes event.pointerType in the ResizeEvent callback, allowing consumers to differentiate between mouse, touch, and pen input.

Suggested by @delijah in #878.

Changes

• Added pointerType?: 'mouse' | 'touch' | 'pen' to ResizeEvent interface
• Pass pointerType through onResizeStart, onResize, and onResizeEnd callbacks
• Updated tests

Usage

<SplitPane
  onResizeStart={(event) => {
    console.log(event.pointerType); // 'mouse', 'touch', or 'pen'
  }}
>

Test Plan

• Tests for mouse pointerType
• Tests for touch pointerType
• Tests for pen pointerType
• All existing tests pass
• TypeScript types updated