feat: add VDefaultsProvider component

[nekosaur]

Description

Motivation and Context

How Has This Been Tested?

Markup:

<template>
  <v-app class="fill-height">
    <v-card title="foo" subtitle="bar" class="ma-10" />
    <v-defaults-provider :defaults="defaults">
      <v-card title="foo" subtitle="bar" class="ma-10" />
    </v-defaults-provider>
  </v-app>
</template>

<script>
  import { ref } from 'vue'

  export default {
    name: 'Playground',
    setup () {
      const defaults = ref({
        global: {
          elevation: 10,
        },
        VCard: {
          color: 'primary',
        },
      })

      return { defaults }
    },
  }
</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)

[johnleider]

Not much I see in the way of changes needed, only chore/doc stuff really.