Automated a11y testing

[myasonik]

Kibana is slowly moving towards integrating automated axe testing with to lint for a11y issues. Automated a11y linting isn't perfect but it does catch ~30% of issues so it's no slouch either.

Could be a great thing to integrate into EUI as well to catch issues earlier in the pipeline (instead of only once components are used in Kibana).

Related: elastic/kibana#43016

Also, our decision making doc on "why axe and not one of the other million tools?" Y'all could pick one of the other tools or one of the many tools that run axe under the hood for you.

[myasonik]

Kibana has merged the first few (7!) automated tests so this can be used as a template for EUI: elastic/kibana#43584