Use content-visibility CSS property to show and hide slides

[yhatt]

content-visibility CSS property in Chrome 85 is the smartest solution for to work slide preloading with avoiding both of issues #153 and #236.

content-visibility: hidden is like a mid state of display: none and visibility: hidden: Elements will be invisible by UA (including user's eye, screen-reader, and search in page), but still calculate the layout in children if needed.
https://drafts.csswg.org/css-contain/#using-cv-hidden

Browser always will skip rendering contents in the element with content-visibility: hidden so the rendering performance has improved as same as using display: none; (#143). Furthermore Marp Core's auto-fitting can use the correct value calculated in "lazy" layout phase (#154).

Marp CLI has made Bespoke's load plugin for detecting auto-fitting elements, but it won't need to use if used content-visibillity: hidden. In addition, annotating aria-hidden to inactive slides through JS (#238) is also not required.