This encourages importing it from `textual.widgets.tree` instead, keeping it
in line with the other changes made for #1637.
Note this is a breaking change.
There's going to be a whole bunch of tests relating to the Tree and TreeNode
coming, let's make sure this ends up being fairly granular.
(side thought: it might be a good time soon to revisit all the tests for
Textual and try and wrangle them into some tidy structure)