Proposal: support keyboard shortcuts to expand/collapse children in TreeView

# Proposal: support keyboard shortcuts to expand/collapse children in TreeView

## Summary
Add support to the TreeView control to expand/recursively expand/collaps the selected node when the +/*/- keys are pressed.

## Rationale
The TreeView controls in WPF (and probably also WinForms/Win32) allow the following keyboard shortcuts when an item is selected in a TreeView:

* plus key: expand selected node
* asterisk key: recursively expand all child nodes
* minus key: collapse selected node

These shortcuts have been in use for a long time and supported across applications. It would be desirable that WinUI continues to support them. Especially the shortcut to recursively expand nodes is sometimes very useful because it is not available via the mouse or touch (as far as I know).

No files, tests, or entry points are named. Start by locating the WinUI TreeView keyboard-input handling, then compare the requested +, *, and - behaviors with existing selection and expansion behavior; done means the selected node expands, recursively expands, or collapses as specified.

Written by the indexing model from the issue text.