Customize Checkbox Icon

Since we have the ability to change switcher icons and node icons it would be nice to change out the checkbox icons with our own components.

I'm able to fake this by hiding the checkbox element and providing my own icon to the node, e.g. (using Material UI icons)

.rc-tree li span.rc-tree-checkbox {
  display: none;
}

then passing in

<TreeNode icon={(props: InternalTreeNodeProps) => {
  if (props.halfChecked) {
    return <IndeterminateCheckbox />
  } else if (props.checked) {
    return <CheckBox />
  }

  return <CheckBoxOutlineBlank />
}} />

This works but prevents us from being able to use custom icons for directories vs leaves, etc.

Ideally it would work identical to the switcherIcon prop:

<Tree
  checkboxIcon={(props: InternalTreeNodeProps) => {
    if (props.halfChecked) {
      return <IndeterminateCheckbox />
    } else if (props.checked) {
      return <CheckBox />
    }

    return <CheckBoxOutlineBlank />
  }}
  switcherIcon={(props: InternalTreeNodeProps) => {
    const { expanded, isLeaf } = props

    if (isLeaf) return null

    return expanded ? <ChevronRight /> : <ExpandMore />
  }}
/>

Start by tracing the existing switcherIcon prop on Tree and TreeNode, then inspect how checkbox rendering currently works. Add an equivalent checkboxIcon customization path that receives the node state shown in the issue, while preserving directory-versus-leaf icons; verify the behavior with the repository's existing tests.