fix(VData): allow group-by to be used with disable-sort

[Esgator]

Description

fixes #11905

Motivation and Context

Currently group-by isn't working properly if disable-sort is set. The group-by algorithm needs a sorted array to work. Usually when group-by is set the array will be sorted first by the group-by criteria then by sort-by criterias.
Now we allow the array to be sorted by group-by criteria although disable-sort is set.

How Has This Been Tested?

playground

Markup:

<template>
  <v-container fluid>
    <v-data-table
      :headers="headers"
      :items="desserts"
      item-key="name"
      group-by="category"
      class="elevation-1"
      disable-sort
    ></v-data-table>
  </v-container>
</template>

<script>
export default {
  data() {
    return {
      headers: [
        {
          text: "Dessert (100g serving)",
          align: "start",
          value: "name",
          groupable: false
        },
        { text: "Category", value: "category", align: "right" },
        { text: "Dairy", value: "dairy", align: "right" }
      ],
      desserts: [
        {
          name: "Frozen Yogurt",
          category: "Ice cream",
          dairy: "Yes"
        },
        {
          name: "Ice cream sandwich",
          category: "Ice cream",
          dairy: "Yes"
        },
        {
          name: "Eclair",
          category: "Cookie",
          dairy: "Yes"
        },
        {
          name: "Cupcake",
          category: "Pastry",
          dairy: "Yes"
        },
        {
          name: "Gingerbread",
          category: "Cookie",
          dairy: "No"
        },
        {
          name: "Jelly bean",
          category: "Candy",
          dairy: "No"
        },
        {
          name: "Lollipop",
          category: "Candy",
          dairy: "No"
        },
        {
          name: "Honeycomb",
          category: "Toffee",
          dairy: "No"
        },
        {
          name: "Donut",
          category: "Pastry",
          dairy: "Yes"
        },
        {
          name: "KitKat",
          category: "Candy",
          dairy: "Yes"
        }
      ]
    };
  }
};
</script>

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Improvement/refactoring (non-breaking change that doesn't add any features but makes things better)

Checklist:

• The PR title is no longer than 64 characters.
• The PR is submitted to the correct branch (master for bug fixes and documentation updates, dev for new features and backwards compatible changes and next for non-backwards compatible changes).
• My code follows the code style of this project.
• I've added relevant changes to the documentation (applies to new features and breaking changes in core library)

[nekosaur]

Could you add a test for this @Esgator

[johnleider]

@Esgator friendly reminder about the unit tests requested by @nekosaur