[6.x] Use entry animations and a base view transition to smooth out Vue loading in

[jaygeorge]

The CP uses a lot of Vue, which causes some jarring layout pop-in.

This PR tries to make things a lot smoother with a mixture of a base view-transition, which mostly handles the exit state (things fading out), mixed with an opacity transition so things "fade in" rather than pop in.

I've used very low timing values, so the page should still feel very quick to load, and you should feel the animation rather than see it.

Some things to note:

• I initially tried to exclusively use view transitions to solve the Vue problem, but they don't seem to be particularly effective when a new page loads.
  • I believe this is due to the delayed page render because of the heavy JavaScript payload.
  • However, a base view transitions does seem to be effective at handling the exit state of the page and wherever we aren't using Vue, such as the sidebar nav
• The most effective way of smoothing out page entries was:
  • Removing the v-cloak from the main container and instead selectively using it on components that were heavier to load. This means the main container that was previously cloaked is no longer blank any more in between pages
  • The skeleton-ui is no longer needed, so I've removed that
  • We're using the newer @starting-style so we can animate on load
  • We're also using the brand new sibling-index()function to stagger animations by a very tiny amount so the load-in feels a little more natural. This is a progressive enhancement