Is your feature request related to a problem? Please describe.
I am trying to deploy a slidev presentation to a subdirectory of GitHub Pages. This requires a hash routerMode. Rather than making a permanent change to the file, it would be good if I could apply this change at build time, similar to how I can specify dark mode or the theme.

Describe the solution you'd like
A new flag --router-mode=hash in slidev build

Describe alternatives you've considered
Manually updating slides.md in the build pipeline seems dangerous.