fix(VBtnToggle): add missing RTL support in button groups

[IwalkAlone]

Description

Make sure that when using VBtnToggle in RTL mode with rounded corners, the correct corners are rounded for the first and last item, and borders are displayed correctly (no missing or doubled borders).

Fixes #11793

Motivation and Context

Fixes #11793

How Has This Been Tested?

visually

Markup:

<template>
  <v-btn-toggle rounded>
    <v-btn>
      <v-icon>mdi-format-align-left</v-icon>
    </v-btn>
    <v-btn>
      <v-icon>mdi-format-align-center</v-icon>
    </v-btn>
    <v-btn>
      <v-icon>mdi-format-align-right</v-icon>
    </v-btn>
    <v-btn>
      <v-icon>mdi-format-align-justify</v-icon>
    </v-btn>
  </v-btn-toggle>
</template>

<script>
export default {
  data: () => ({}),
  mounted() {
    this.$vuetify.rtl = true;
  },
};
</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)

[jacekkarczmarczyk]

lgtm