Feature request

What problem does this feature solve?

I am working on a site which, for some text filtering reason, requires htm instead of html as the
extension of output HTML pages. vuepress-plugin-clean-urls kind of solves this problem by changing the output path, but internal links that ends with .md seems to be hardcoded to be replaced by .html, which is not ideal.

What does the proposed API look like?

Probably a option such as htmlSuffix, or a new plugin?

How should this be implemented in your opinion?

Same as above.

Are you willing to work on this yourself?

I am willing to try to work on this, but it seems that this is hardcoded in an unknown place (I tried debugging @vuepress/markdown/lib/link.js, but no breakpoint is hit.