fix(VTreeview): independent selection inheriting parent state

[ThomasKientz]

Description

fixes #14955

Motivation and Context

How Has This Been Tested?

visually

Markup:

<template>
  <v-app id="inspire">
    <v-main>
      <v-btn
        color="info"
        @click="setSelected"
      >select last added</v-btn>
      <!-- selection-type="independent" -->
      <v-treeview
        v-model="selectedItems"
        dense
        selectable
        :items="items"
        :load-children="loadChildren"
      ></v-treeview> </v-main></v-app>
</template>

<script>
  export default {
    data: () => ({
      selectedItems: [],
      ctr: 0,
      items: [
        {
          id: 1,
          name: '1',
          children: [
            { id: 2, name: '2', children: [] },
            { id: 3, name: '3', children: [] },
            { id: 4, name: '4', children: [] },
          ],
        },
      ],
    }),
    methods: {
      setSelected () {
        this.selectedItems.push('new_' + (this.ctr - 1))
      },
      loadChildren (node) {
        const id = 'new_' + this.ctr++
        node.children.push({ id, name: id, children: [] })
      },
    },
  }
</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)