fix: better nav types

[zRains]

Thanks for solving my problem before, but here I think a little change is needed.

VPMenuGroup.vue only one level of nesting is allowed：

<template>
  <div class="VPMenuGroup">
    <p v-if="text" class="title">{{ text }}</p>

    <template v-for="item in items">
      <VPMenuLink v-if="'link' in item" :item="item" />
    </template>
  </div>
</template>

But the type items: NavItem[] may be infinitely nested, the following Nav configuration code is allowed:

nav: [
    {
        text: 'Dropdown Menu',
        items: [
            {
                // Title for the section.
                text: 'Section A Title',
                items: [
                    { text: 'Section A Item 1', link: '...' },
                    { text: 'Section A Item 2', link: '...' },
                ],
            },
            {
                // Title for the section.
                text: 'Section B Title',
                items: [
                    { text: 'Section B Item 1', link: '...' },
                    {
                        text: 'Section B Item 2',
                        items: [
                            {
                                // Title for the section.
                                text: 'Section C Title',
                                items: [
                                    { text: 'Section C Item 1', link: '...' },
                                    { text: 'Section C Item 2', link: '...' },
                                ],
                            },
                        ],
                    },
                ],
            },
        ],
    },
]

It passed the type check.