Add more a11y tests via axe-core

[mfranzke]

To prevent problems like e.g. db-ui/core#820 we should further optimize our testing and linting.

see also #2579

[nmerget]

https://github.com/marketplace/actions/lighthouse-ci-action

or

https://github.com/GoogleChrome/lighthouse-ci/blob/main/docs/getting-started.md#prerequisites

[nmerget]

I tested with lighthouse ci, and it takes very long in the CI.

Currently, we only need the accessibility feature which is provided by axe-core, which is already in our repo via playwright.
I will rename this issue and include axe to our showcase tests to fix those problems.

[mfranzke]

I tested with lighthouse ci, and it takes very long in the CI.

Currently, we only need the accessibility feature which is provided by axe-core, which is already in our repo via playwright. I will rename this issue and include axe to our showcase tests to fix those problems.

I meant to use lighthouse not just for a11y, but also the other nice included test types, but let's focus on a11y first of all. I've added a new lighthouse issue to follow up with independently: #2582