feat(tree): modify children of existing `Tree`

[bashbunni]

This allows users to modify existing Trees. In the current version of this table, there is no way to change existing children outside of adding a new child with Tree.Child().

This also handles operations on Leaf types to turn them into Trees.

See examples for usage on these changes. Those examples are testable, so see their output in comment to see how these API changes work.

Related: https://github.com/charmbracelet/lipgloss/discussions/452

[bashbunni]

Need to look at the feedback from https://github.com/charmbracelet/lipgloss/discussions/452 more closely