fix(VListItem): dont trigger kb events for disabled item

[lampdev]

Description

Add check for disabled prop to keydown handler

Motivation and Context

Fixes #15322

How Has This Been Tested?

unit and visually

Markup:

<template>
  <v-container>
    <p class="pa-6 text-center">The second item should not emit the click event when focussed using keyboard and pressing enter:</p>
    <v-card class="mx-auto" max-width="400" tile>
      <v-list-item tabindex="0" @click="ClickMe()">
        <v-list-item-content>
          <v-list-item-title>Single-line item</v-list-item-title>
        </v-list-item-content>
      </v-list-item>

      <v-list-item disabled tabindex="0" two-line @click="ClickMe()">
        <v-list-item-content>
          <v-list-item-title>Two-line item</v-list-item-title>
          <v-list-item-subtitle>Secondary text</v-list-item-subtitle>
        </v-list-item-content>
      </v-list-item>

      <v-list-item tabindex="0" three-line>
        <v-list-item-content>
          <v-list-item-title>Three-line item</v-list-item-title>
          <v-list-item-subtitle>
            Secondary line text Lorem ipsum dolor sit amet,
          </v-list-item-subtitle>
          <v-list-item-subtitle>
            consectetur adipiscing elit.
          </v-list-item-subtitle>
        </v-list-item-content>
      </v-list-item>
    </v-card>
  </v-container>
</template>

<script>
  export default {
    data: () => ({
    //
    }),
    methods: {
      ClickMe() {
        alert("Clicked");
      }
    }
  }
</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)