feat(VDataTable): add isOpen to group.header slot

[Daanra]

Description

Adds 'isOpen' as a prop to the group.header slot.

Motivation and Context

Currently, the toggle function is passed as a prop to the group.header slot. However, when utilizing this method, you probably want to display a different icon depending on whether the header is opened or not (as is happening in the default implementation).

How Has This Been Tested?

Visually (playground)

Markup:

<template>
  <v-container>
    <v-data-table :items="items" :headers="headers" group-by="type">
      <template #group.header="{ isOpen, toggle }">
        <v-btn @click="toggle" icon>
          <v-icon>
            {{ isOpen ? '$minus' : '$plus' }}
          </v-icon>
        </v-btn>
      </template>
    </v-data-table>
  </v-container>
</template>

<script>
export default {
  data: () => ({
    items: [{
      type: 1,
      name: 'First',
    }, {
      type: 2,
      name: 'Second',
    }, {
      type: 1,
      name: 'Third'
    }],
    headers: [{
      text: 'Type',
      value: 'type'
    }, {
      text: 'Name',
      value: 'name',
    }]
  })
}
</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 breaking 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)
• I've added new examples to the kitchen (applies to new features and breaking changes in core library)

[nekosaur]

update the api-generator map file as well

[johnleider]

Do we have a baseline naming convention for active state? I don't think we use isOpen anywhere.