[Feature request] Custom date format of `lastUpdated`

[narumi147]

Feature request

Description

Here are the default datetime format:
CN: 2021/10/14 下午6:42:10
EN: 10/14/2021, 6:42:10 PM

I actually only want to show date and hide time such as 2021/10/14, 10/14/2021

It's better if we can custom date format.

[narumi147]

https://github.com/vuepress/vuepress-next/blob/edecf2d2b0735200b2b354afabc851ebf43e3225/packages/%40vuepress/theme-default/src/client/components/PageMeta.vue#L102

For example, add such a option dateOnly, and use toLocaleDateString() instead of toLocaleString() if that's true.

[github-actions]

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