Downgrade bundlesize until they make brotli optional since it doesn't work with node.js 10.x yet.

So, babel, qunit/karma-qunit and bundle-size aren't the latest versions.

Among other things, closes https://github.com/twbs/bootstrap/issues/26548

By applying the transition: CSS property only to classes that are
active during sliding, we avoid an unnecessary, non-zero-time
animation that although mostly invisible, does interfere with e.g.
z-index based parallax

H3 was used without preceding H2, breaking document flow.

* Doc update - remove media-breakpoint-up(xs)

If media-breakpoint-down(xl) is not listed, then media-breakpoint-up(xs) does not need to be listed either.
The above 4 media queries are now aligned with the 4 media-breakpoint-up mixins (just like their media-breakpoint-down counterparts).

* Improve media-breakpoint-up/down docs

Also, move the search code to a separate file.

for clarity/consistency with other similar examples

`<dialog>`, in browsers that support it, has user agent styles of `dialog { display:block; ...} dialog:not([open]) { display:none; }`
by forcing it to `display:block` in the shim, the dialog is shown even when closed. There's no clean way to shim this for non-supporting browsers,
but arguably these browsers would have further problems with pure `<dialog>` usage anyway, and it's up to authors to then use different elements/shims.