Create plugin: row grouping

[NullVoxPopuli]

This plugin would provide an alternate API that consumers would use to enumerate their rows. Potentially something like:

type Rows = Array<
  | Row 
  | { 
      groupName: string,
      ...otherProperties,
      rows: Row[]
    }
>;

To access these groups, a consumer would need to do something like:

import { meta } from 'ember-headless-table/plugins';
// ...
meta.forTable(table, RowGrouping).rows;

This could be abstracted away from consumers via a helper function:

export const groupedRows = (table) => meta.forTable(table, RowGrouping).rows;
export const isGroup = (row: Row | RowGroup) => 'groupName' in row;

Which could then be used in a template like this:

import { groupedRows, isGroup } from '...';

<template>
  {{! ... }}
  {{#each (groupedRows table) as |row|}}
    {{#if (isGroup row)}}
      markup tbd

         {{#each row.rows as |row|}}
             group content here
         {{/each}}

    {{else}}
      <tr>
          normal rows
      </tr>
    {{/if}}
  {{/each}}
</template>