fix(VTreeview): stuttering animation of toggle icon when loading

[jacekkarczmarczyk]

Motivation and Context

fixes #9475

How Has This Been Tested?

playground

Markup:

<template>
  <v-container>
    <div class="v-icon v-treeview-node__toggle mdi mdi-cached v-treeview-node__toggle--loading" />
    <div class="v-icon mdi mdi-cached v-treeview-node__toggle--loading" />

    <v-divider />

    <v-treeview :load-children="fetch" :items="items" open-on-click selectable />
  </v-container>
</template>

<script>
export default {
  data: () => ({
    items: [{ id: 1, name: 'All Breweries', children: [] }],
  }),

  methods: {
    async fetch () {
      await new Promise((resolve) => { setTimeout(() => resolve(), 5000) })
      this.items[0].children = [{ id: 2, name: 'Some Brewery', 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 breaking 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)
• I've added new examples to the kitchen (applies to new features and breaking changes in core library)