Add index to Tree widget's add

[davetapley]

Presently there is no way to add a new tree node except as the last node, accordingly:

Would you accept an optional indexhere? https://github.com/Textualize/textual/blob/ce962196c73b2b87c9f812f66a6f6ded3f2aaf4b/src/textual/widgets/_tree.py#L320-L327

If set it would change this from append to insert and pass index: https://github.com/Textualize/textual/blob/ce962196c73b2b87c9f812f66a6f6ded3f2aaf4b/src/textual/widgets/_tree.py#L344

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• How do I center a widget in a screen?
• Does Textual support images?

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[willmcgugan]

Would that cover all use-cases? What if you want to insert before another item, or at the start.

The mount method has a before and after parameter. Maybe we need something similar.

[davetapley]

@willmcgugan it would, but the caller would be responsible for figuring out the index.