feat(style): add font-weight semibold helper class

[FraCata00]

Description

Add missing font weight css class directtive for font-weight 600
resolves #20585
helped by @umbertopaolasso

Markup:

The same as Contributing guide as follow https://vuetifyjs.com/en/getting-started/contributing/#local-development

No custom setup environment

<template>
  <v-app>
    <v-container>
      <div>
        <p class="font-weight-black">
          Black text.
        </p>
        <p class="font-weight-bold">
          Bold text.
        </p>
        <!-- new class -->
        <p class="font-weight-semibold">
          Semibold text.
        </p>
        <p class="font-weight-medium">
          Medium weight text.
        </p>
        <p class="font-weight-regular">
          Normal weight text.
        </p>
        <p class="font-weight-light">
          Light weight text.
        </p>
        <p class="font-weight-thin">
          Thin weight text.
        </p>
        <p class="font-italic">
          Italic text.
        </p>
      </div>
    </v-container>
  </v-app>
</template>

<script>
  export default {
    name: 'Playground',
    setup () {
      return {
        //
      }
    },
  }
</script>