Description of the new feature/enhancement

In nvaccess/nvda#13671 (comment) @carlos-zamora thought that wt and conhost should implement movement, expansion, etc. by the UIA "page" unit. This would help NVDA to solve nvaccess/nvda#13157 and improve the UX for other UIA clients.

Proposed technical implementation details (optional)

The "page" unit should represent one visible screen of text, and behave similar to Microsoft Word's implementation. In other words:

1. Run the "less" command, or anything else that paginates output.
2. Jump to the end of the text. For iinstance, create a document range and "collapse" it by moving its start to its end, creating a degenerate range.
3. Move this new degenerate range back one page. It should now be a degenerate range at the start of the currently visible screen of output.
4. Expand to enclosing unit. The range should now span the visible output.
5. Move to the previous page. The range should now contain the last visible screen of output.