fix(VSnackbar): a11y and appearance improvements

[phiter]

Description

This merge request improves the appearance of the Snackbar action button and makes the whole snackbar be properly announced by screen readers.

Motivation and Context

Focus states are obligatory for accessibility and the current action button does not have a focus state.
Also the screen reader should immediately announce a snackbar message when it shows up.
fixes #10047

How Has This Been Tested?

I have tested using NVDA and visually too.

Markup:

<template>
  <v-container>
    <v-btn
      dark
      @click="snackbar = true"
    >
      Open Snackbar
    </v-btn>

    <v-snackbar
      v-model="snackbar"
    >
      {{ text }}
      <v-btn
        text
        @click="snackbar = false"
      >
        Hello
      </v-btn>
    </v-snackbar>
  </v-container>
</template>

<script>
  export default {
    data: () => ({
      snackbar: false,
      text: 'Hello, I am a snackbar',
    }),
  }
</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)