fix: allow explicit scoped slot binding via template

This allows mounting options like:

scopedSlots: {
  'group-label': `<template #group-label="{ group }"><em>Group: {{ group.name }}</em></template>`,
  'list-item': `<template #list-item="{ item }"><em>Item: {{ item.text }}</em></template>`,
},

This works consistently in VTU@1 and VTU@2, so it should be allowed.

See gitlab-org/gitlab-ui!4901 (comment 2285518696) for context.