ui.table selection_mode, selection_area

From the spec in the DESIGN doc: https://github.com/deephaven/deephaven-plugins/blob/main/plugins/ui/DESIGN.md#parameters-17

```
selection_mode: 	SelectionMode | None 	# Can be MULTIPLE to allow multiple selection or SINGLE to not allow it.
selection_area: 	SelectionArea | None 	# The unit that is selected on press. Can be ROW, COLUMN, or CELL.
```