Remove aria attributes and role from all slots

[claviska]

Slots with aria and/or role attributes are invalid:

• https://w3c.github.io/html-aria/tests/slot.html
• https://developer.mozilla.org/en-US/docs/Web/HTML/Element/slot#technical_summary

They cause problems in assistive devices and they cause Lighthouse's accessibility checker to error out.

What we need to do

• Add a lint rule to prevent regressions and future incidents. Since this isn't specific to Shoelace, it's best to do this upstream in the eslint-plugin-lit-a11y plugin. Much of the work has already been done upstream, but we need to contribute:
  • Additional logic to check for <slot role="">
  • Add documentation (see TODOs)
  • Add test cases (see TODOs)
• When open-wc releases it, update the eslint-plugin-lit-a11y plugin in Shoelace
• Remove all aria attributes from slots in the Shoelace codebase

Special thanks to @thepassle for scaffolding out the PR to make this possible!

[claviska]

I've made the PR upstream and it was released today. I have a local branch in progress handling this. We have some failing lint rules that I'm still working out.