Tweaks to DataTable generics (#1968)

Adjust DataTable types.
- Make key in `cell` slot prop less strict to prevent type errors in markup.
- Resolve property path names up to one level deep for header keys.

tests/DataTableAppendColumns.test.svelte

@@ -54,11 +54,7 @@
   ];
 </script>
 
-<DataTable
-  sortable
-  headers="{headers}"
-  rows="{rows}"
->
+<DataTable sortable headers="{headers}" rows="{rows}">
   <span slot="cell" let:cell>
     {#if cell.key === "overflow"}
       <OverflowMenu flipped>