fix(VMenu): allow more than 1 content node

[jacekkarczmarczyk]

Motivation and Context

fixes #10522

related cc17fe1

How Has This Been Tested?

playground, unit

Markup:

<template>
  <v-container>
    <v-subheader>VEditDialog</v-subheader>
    <v-edit-dialog large>
      <template #input><span>foo</span><span>bar</span></template>
      <span>foo</span><span>bar</span>
    </v-edit-dialog>

    <v-subheader>VMenu</v-subheader>
    <v-menu
      offset-y
      content-class="white"
    >
      <template #activator="{ on }">
        <v-btn v-on="on">Menu</v-btn>
      </template>
      <div>Foo</div>
      <div>Bar</div>
    </v-menu>

    <v-subheader>VTooltip</v-subheader>
    <v-tooltip bottom>
      <template #activator="{ on }">
        <v-btn v-on="on">Tooltip</v-btn>
      </template>
      <div>Foo</div>
      <div>Bar</div>
    </v-tooltip>

    <v-subheader>VWindow</v-subheader>
    <v-window>
      <v-window-item>
        <span>foo</span>
        <span>bar</span>
      </v-window-item>
    </v-window>

    <v-subheader>VExpansionPanels</v-subheader>
    <v-expansion-panels>
      <v-expansion-panel>
        <v-expansion-panel-header>
          <span>foo</span><span>bar</span>
        </v-expansion-panel-header>
        <v-expansion-panel-content>
          <span>foo</span><span>bar</span>
        </v-expansion-panel-content>
      </v-expansion-panel>
    </v-expansion-panels>

    <v-subheader>VCarousel</v-subheader>
    <v-carousel height="100">
      <v-carousel-item>
        <span>foo</span>
        <span>bar</span>
      </v-carousel-item>
    </v-carousel>

    <v-subheader>VListGroup</v-subheader>
    <v-list>
      <v-list-group>
        <template v-slot:activator>
          <v-list-item-content>
            <v-list-item-title>foo</v-list-item-title>
          </v-list-item-content>
        </template>
        <v-list-item>bar</v-list-item>
        <v-list-item>baz</v-list-item>
      </v-list-group>
    </v-list>
  </v-container>
</template>

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)

[johnleider]

Looks good to me, would like a final approval from @KaelWD since it has to do with his previous commit.

[KaelWD]

Um yeah I made some big changes to this PR so I was expecting someone else to review it.