Issue #1775774 by attiks, effulgentsia: Added Allow themes to identify their breakpoints to Drupal.

core/themes/bartik/config/bartik.breakpoints.yml

+mobile: '(min-width: 0px)'
+narrow: 'all and (min-width: 560px) and (max-width:850px)'
+wide: 'all and (min-width: 851px)'

core/themes/seven/config/seven.breakpoints.yml

+mobile: '(min-width: 0em)'
+wide: 'screen and (min-width: 40em)'

core/themes/stark/config/stark.breakpoints.yml

+mobile: '(min-width: 0px)'
+narrow: 'all and (min-width: 480px) and (max-width: 959px)'
+wide: 'all and (min-width: 960px)'