web: add redesign-theme alert styles

Created by: valerybugakov

Description

This PR updates our .alert elements to use the new refresh styles. Closes #19662 (closed).

Changes

• Updated alert styles under .theme-redesign.
• Extracted AlertsStory in a separate component.
• Added color-scheme-mixins for a repetitive pattern that we use to apply certain rules only for .theme-light or .theme-dark.

Some components from the Chromatic changeset need to be updated. See #20666 (closed). It will be done in a separate PR because changes are required only for the redesign theme.

Storybook