VaDataTable: change track-item-by to track-by and add value-by

[m0ksem]

Similar to VaSelect, need props like track-by and value-by for selected items when working with selectable data table.

      <VaDataTable 
         selectable
         v-model="selectedItems"
         :items='items'
         :columns='[{ key: "name", sortable: true }]'
         itemsTrackBy="id"
      >

Maybe we can tweak or improve useSelectable. We don't need text-by thought.

I think we need to have useSelectable (track-by, value-by primitive) <- useSelectableList (text-by, disabled-by, group-by etc).

Also make sure to add comments to this composable, because it is not obvious thing for new devs.