support activeMatch for sidebar items, or match on URL hash

[josefaidt]

Is your feature request related to a problem? Please describe.

When creating a sidebar structure from anchor points in a single document (think a list of commands on a single page), Vitepress does not highlight the active anchor, and instead highlights the sidebar item clicked to navigate to the page (e.g. clicking on help navigates to /commands/#help)

help -> /commands/#help

Describe the solution you'd like

• support URL hash for sidebar active match
• expose activeMatch with behavior similar to DefaultTheme.NavItem

Describe alternatives you've considered

n/a

Additional context

I may be able to implement this feature

Validations

• Follow our Code of Conduct
• Read the docs.
• Read the Contributing Guidelines.
• Check that there isn't already an issue that asks for the same feature to avoid creating a duplicate.