Feature request: column grouping

[NullVoxPopuli]

Not dissimilar to #20

group a		group b
sub a a	sub a b	sub b a	sub b b
data a	data b	data c	data d

configuration could look like this:

import { defineColumns, GroupedColumns } from 'the-group-plugin';
// ...
headlessTable({
  columns: () => defineColumns([
    {
      groupName: 'group a',
      columns: [...]
    },
   {
      groupName: 'group b',
      columns: [...]
    }
   ]),
  plugins: [GroupedColumns, ColumnReordering, ColumnResizing]
});

I think the existing column Re-ordering and Column re-sizing plugins should still work with grouped columns.