fix(VListItem): properly display active-color when active

[nekosaur]

Description

fixes #13893 with computed instead of reactive

Motivation and Context

keeps code base consistent, and adds test

How Has This Been Tested?

cy test and playground

Markup:

<template>
  <v-app>
    <v-list
      lines="three"
      max-width="500"
      theme="dark"
    >
      <v-list-item
        v-for="n in 3"
        :key="n"
        title="Three line item"
        :active="n === model"
        subtitle="lorem"
        active-color="success"
        text="text"
        @click="model = n"
      />
    </v-list>
  </v-app>
</template>

<script>
  export default {
    name: 'Playground',
    data: () => ({
      model: 1,
    }),
  }
</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)