[Feature request] CleanUrl

[Mister-Hope]

Clear and concise description of the problem

What's your attitude about supporting clean url? E.g.: /a.html becomes /a

Although this could be a plugin, but I believe supporting it in core could be better? (The path on Page object is "clean" always)

Suggested solution

A new option router in vuepress config, containining cleanUrl.

Alternative

No response

Additional context

No response