[D8] Add Breakpoints in core

Description of the need

I wonder if we should consider adding breakpoint functionality to core as D8 has done. This would allow core to manage breakpoints that could be used by modules, themes, layouts, ...

I started work on porting that module from D7: - https://github.com/backdrop-ops/contrib/issues/409 - https://github.com/backdrop-contrib/breakpoints

Here's the D8 info: - https://www.drupal.org/docs/theming-drupal/working-with-breakpoints-in-d...

This was @klonos in a separate thread:

Well, we are aiming for as much feature parity with D8 as possible (where it makes sense, and for features that would make people hesitate choosing Backdrop over D8), so I'd be all in for a breakpoints API in core, that both core and contrib could then use.

Breakpoints in core would help with make developing complex frontend themes easier, but could also be used in responsive things in the admin UI, like the admin bar, the primary tabs, the "low priority" columns in tables etc. Breakpoints could even be used as a visibility condition for blocks.