Text Search on ComboBox

As users type characters on a physical keyboard while focused on an open or closed combo box, candidates matching the user's string are brought into view. This functionality is especially helpful when navigating a long list. For example, when interacting with a drop-down containing a list of states, users can press the "w" key to bring "Washington" into view for quick selection. The text search is not case-sensitive.

**Additional context**
- https://docs.microsoft.com/en-us/windows/apps/design/controls/combo-box#text-search

The issue names no files or tests. Start by locating the ComboBox implementation and its keyboard-handling entry point, then verify case-insensitive text search while the control is open and closed. Done means typing characters brings matching candidates into view, including items in a long list.

Written by the indexing model from the issue text.