Allow getReferenceClientRect to be overridden

Prefer using getReferenceClientRect in tippyOptions if available.
This allows modifying the position of the bubble menu in a different
location for block level elements, like above a table cell or a code block.

packages/extension-bubble-menu/src/bubble-menu-plugin.ts

@@ -180,7 +180,7 @@ export class BubbleMenuView {
     }
 
     this.tippy?.setProps({
-      getReferenceClientRect: () => {
+      getReferenceClientRect: this.tippyOptions.getReferenceClientRect || () => {
         if (isNodeSelection(state.selection)) {
           const node = view.nodeDOM(from) as HTMLElement