Add navigation slots

[stafyniaksacha]

Is your feature request related to a problem? Please describe.

With the new theme, we can not easily add content in the navbar via slots as we can do for the home and aside parts

Describe the solution you'd like

It would be great to have those slots exposed in the Layout:

• nav-bar-title-before
• nav-bar-title-after
• nav-bar-content-before
• nav-bar-content-after
• nav-screen-content-before
• nav-screen-content-after

https://github.com/vuejs/vitepress/blob/next-theme/src/client/theme-default/Layout.vue#L27

Describe alternatives you've considered

No response

Additional context

No response

Validations

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