feat: smooth scroll and prefs

[userquin]

POC: by default smooth scroll not enabled and we can remove it from the configuration, enabled by default (I need to change the docs).

From the comments here https://github.com/vuejs/vitepress/pull/978#issuecomment-1183549569, it seems there are some caveats with the implementation in all browsers:

• chrome: just works, but scrolling from the right aside links seems to not work; from the left aside seems to work
• firefox: ~~not working; we can add a manual focus using scrollIntoView in the right aside, we'll need to include behavior: 'smooth' (tested but removed, see safari)~~ check next comment
• safari: smooth not supported

[userquin]

It seems Firefox is using smooth scroll when the target is close, if the distance exceeds some threshold it will apply normal behavior.

EDIT: why not just open an issue on Chrome?

[kiaking]

Closing due to inactivity. The PR becoming obsolete 😓