feat(VTimePicker): control selecting mode with selecting

[J-Sek]

Description

New prop lets VTimePicker propagate access to current "selecting mode" and might open more possibilities (e.g. [1] not fully functional without #20200)

resolves #20671

Markup:

<template>
  <v-app>
    <v-container>
      <div class="d-flex justify-center ga-12"></div>
      <v-row class="justify-center ga-10" no-gutters>
        <v-card class="pa-6">
          <v-time-picker
            v-model="time"
            ampm-in-title
            use-seconds
            scrollable
            v-model:selecting="selecting"
          />
        </v-card>
        <v-card class="pa-6" width="300">
          <pre class="mb-6">value: <b>{{ time }}</b></pre>
          <v-radio-group v-model="selecting">
            <v-radio label="Hour" :value="1" />
            <v-radio label="Minute" :value="2" />
            <v-radio label="Second" :value="3" />
          </v-radio-group>
        </v-card>
      </v-row>
    </v-container>
  </v-app>
</template>

<script setup>
  import { ref } from 'vue'
  const time = ref('11:15')
  const selecting = ref(1)
</script>