added SetItemHeight to the Tree widget, copying from the List widget

[kothawoc]

Description:

This adds SetItemHeight to the Tree widget. It is copied from the List widget to keep it similar.

I require dynamic heights for items in the tree, which wasn't supported, this implements them using the same public API as the List widget.

Fixes #(issue)

Checklist:

• Tests included.
• Lint and formatter run with no errors.
• Tests all pass.

Where applicable:

• Public APIs match existing style and have Since: line.
• Any breaking changes have a deprecation path or have been discussed.
• Check for binary size increases when importing new modules.

[andydotxyz]

@kothawoc are you still looking to take this forward? it would be a great addition for v2.6.0 but it's currently missing tests.