fix(VBtn): correct font-size for different VBtn sizes

[obennaci]

fix #8358

Description

Motivation and Context

How Has This Been Tested?

Playground

Markup:

<template>
  <v-container>
    <v-btn x-small>X-SMALL</v-btn>
    <v-btn small>SMALL</v-btn>
    <v-btn>DEFAULT</v-btn>
    <v-btn large>LARGE</v-btn>
    <v-btn x-large>X-LARGE</v-btn>
  </v-container>
</template>

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)

[jacekkarczmarczyk]

This is current PR:

How about this sizes?

$btn-font-sizes: (
  'x-small': .625rem,
  'small': .75rem,
  'default': .875rem,
  'large': 1rem,
  'x-large': 1.125rem
) !default;

[obennaci]

I've only replicated the small and large sizes from v1.5

Tried the suggested sizes and they seem better overall, let me know if I should update them.

[johnleider]

Go ahead and make the suggested changes, looks good.