Add ability to render native drag preview in context of specific element.

[olivercoad]

When generating the custom drag preview, the container is added directly to body. This means it inherits the CSS state of body, including CSS variables, font, currentColor etc.

I'd love the option to provide an alternative element to document.body to mount the container to, so that it can inherit the desired CSS state from the context of where the dragged element is.

The main reason I want this is so that it can get use the css variables from the theme, which aren't present on the body.

Because container is position: fixed, it will still render in the correct place.
Potential gotchas to look out for are when the element:

• is hidden or display: none
• has opacity
• has other things above it in the stacking context

Proposed change

export function setCustomNativeDragPreview({
	render,
	nativeSetDragImage,
	getOffset = defaultOffset,
        containerParent = document.body,
}: {
	getOffset?: GetOffsetFn;
	render: RenderFn;
	nativeSetDragImage: ElementEventPayloadMap['onGenerateDragPreview']['nativeSetDragImage'];
        containerParent?: HTMLElement;
}) {

/* ... */
        // document.body.append(container);
	containerParent.append(container);

/* cleanup */
            // document.body.removeChild(container);
            containerParent.removeChild(container);