[two_dimensional_scrollables] Enable multi-cell/single-cell text selection in Simple Table example (#8189)
This change adds a SegmentedButton to enable different variations of text selection for the example.
* Multi-cell selection, enables selection across the entire table, allowing you to select across cells.
* Single-cell selection, enables selection on individual cells, so the selection cannot escape the cell it started in.
* Disabled, disables selection across the entire table.
Text selection in a table is a common use-case that we should showcase.