Support browse mode table navigation in Microsoft Edge (#6617)
* Abstract virtualBuffer table navigation support so it can be available in all browse mode documents.
If an API has the ability to look up a cell by row and column (and correctly handles row/col spans) then only BrowseModeDocumentTreeInterceptor._getTableCellAt needs to be overridden. For more complex cases, BrowseModeDocumentTreeInterceptor._getnearestTableCell may have to be overridden.
* Support browse mode table navigation for UI Automation (such as Microsoft Edge).
