Support custom date format or date only for lastUpdated

[narumi147]

Raised 2y ago, but no response.

• [Feature request] Custom date format of lastUpdated #476

https://github.com/vuepress/vuepress-next/blob/db3ce2cc5fcee113cdd16d84c89d08fba2f6929e/ecosystem/theme-default/src/client/components/PageMeta.vue#L69

Support format callback would be pretty nice.

[Mister-Hope]

pr welcome

[narumi147]

also tried once, but lack js experience (；へ：)