Use content-visibility rather than display: none to hide <details> co…

…ntent.

This matches the current shipping version of Chromium, and the last
version of the spec, except for the <slot> display when open, which per
spec ought to be `block`, but it's still `contents`.

animation-canceled-by-parent-details-element-being-closed.html is
invalid because content-visibility doesn't cancel animations, just
pauses them. It also times out in Chrome.

Differential Revision: https://phabricator.services.mozilla.com/D217192

bugzilla-url: https://bugzilla.mozilla.org/show_bug.cgi?id=1789166
gecko-commit: 6904d078df0be1cd17df43b3ed219a17708386b8
gecko-reviewers: smaug