feat(theme-default): subtitle automatically appears in the view when it overflows the visual area

[zh-lx]

close #658
When I scroll markdown content, I need to swipe sidebar to see the corresponding subtitle if the current subtitle is too far behind：

This PR will realize the function that the page content scrolls, if the active subtitle overflows the visual area, it will automatically scroll to the visual area. The effect is shown in GIF below:

[Mister-Hope]

Wait... Does it work with sidebar group or even nested sidebar group? Since each sidebar group will have a sideLinks as children, there might be mutiple components in a single sideabr

[zh-lx]

Wait... Does it work with sidebar group or even nested sidebar group? Since each sidebar group will have a sideLinks as children, there might be mutiple components in a single sideabr

Yes, I have tested the case, there might are many active sidebars but only one of them whose href is completely match both path and hash of the page. And the code will find this one and let it scrollIntoView.

const activeSidebarItem = document.querySelector(
  `[href="${router.currentRoute.value.path}${router.currentRoute.value.hash}"]`
)