Allow to specify the key prop in the TreeNodeData type

[alessandrojcm]

Hi there!

Long time Mantine user here, really loving the new Tree component, it came it handy just at the right time. I was just using it today and I realized the key prop for the tree nodes are inferred from the prop value. I was having this weird bug there clicking a leaf node will collapse its parent, I then realized the node.

Now I understand that this was my fault as I was not ensuring the node values were unique, however I believe it would be a good idea (unless there is something I am missing) to expose the key prop to allow users to manually set their keys, and it could default to the node value. Happy to make this PR, too, if that's ok.

Thanks!

[rtivital]

This issue is not related to key prop in React. Actions on tree items are performed based on their value. You can treat value as key in the current implementation – it must be unique.