Add automatic accessibility testing add-on to storybook

[thrbnhrtmnn]

Description / User Story

We should integrate some automatic accessibility testing add-on to storybook. Before we choose an add-on we need to do some research

---

Acceptance Criteria

• Research was done to define which add-on we want to implement to storybook
• Decision was made which add-on to implement
• Add-on was implemented
• All issues that were found have been documented per component and prioritised for the follow-up task Fix critical accessibility issues #595
  • TextButton
  • IconButton
  • Loader
  • Checkbox
  • Radio
  • RadioGroup
  • TextArea
  • TextInput
  • Select
  • ToggleSwitch
  • Counter
  • FormCaption
  • FormCaptionGroup
  • FormLabel
  • ButtonGroup
  • Divider
  • NumberInput
• For later (follow up task):
  • [ ] Tooltip (Tooltip - refactor component to use floating-ui #461 )

---

Background information

• One example of such an add-on: https://www.kickstartds.com/storybook/?path=/story/base-button--solid (see canvas and accessibiltiy tab)
• The checks will not be done for Slider and TabBar, as these components will be changed substantially again with TabBar - use slots instead of arrays in the Tabbar #513 and Slider - tokenise component and finalise design #268.

[thrbnhrtmnn]

Today we already implemented an add-on / plug-in, the findings still need to be documented in the follow-up task though

[thrbnhrtmnn]

This task is now mostly done. I propose to skip slider and tabbar, but to still wait for tooltip and numberinput as well as update of formcaptiongroup, to then also check these components again.

[thrbnhrtmnn]

Follow-up task: Check Accessibility issues of Tooltip and add them to Fix Ticket