Remove `selectedRow` prop from `DataTable` and add a new prop to toggle multi-row selection

https://github.com/hypothesis/frontend-shared/pull/1360 introduced support for multi-row selection, by adding a new `selectedRows` prop.

With those changes, `DataTable` allows multi-row selection only if `selectedRows` is provided, and single-row selection if `selectedRow` is instead. However, we have not explicitly handled what happens if both are provided, other than just preferring `selectedRows` over `selectedRow`.

To make the props API more obvious, let's remove `selectedRow`, and add a new prop to enable/disable multi-row selection.