Feature: Add $deepCopy util function to public API #9898

Description

In various tests, a $deepCopy function is declared; its purpose is to perform a deep clone of a node, including its slots (and potentially nodes that possess excludeCopy methods?), given that the existing $copyNode function does not handle this.

I suggest exposing this function in the public API, as it could be useful both in tests and in user code. You should also write about this function in the documentation. Also this function should be mentioned in the documentation https://lexical.dev/docs/concepts/node-cloning

Impact

Reduces boilerplate code in tests and adds the ability to implement a "Duplicate" feature for elements, similar to what is found in some popular editors