Use subtheme name as breakpoint group

Problem/Motivation

Currently the subtheme's breakpoints.yml file has "screen" as the name of the group the breakpoints belong to. This can be a little confusing as groups are usually provided by themes. For example, see the screenshot for how the Responsive Image config screen lists them.

Steps to reproduce

Install theme, create subtheme, enable Responsive image module and review the list of breakpoint groups in the dropdown.

Proposed resolution

Use the subtheme name as the default breakpoint group name.

Additional breakpoint groups can be added manually.

Remaining tasks

It is simple as changing the "group" from screen to SUBTHEME.screen rather than screen in the yml file. See corresponsiding docs: https://www.drupal.org/docs/theming-drupal/working-with-breakpoints-in-d...

User interface changes

The sub theme's breakpoint group would now be the name of the theme.

API changes

It shouldn't affect any existing subthemes, only new ones.

Data model changes