Add an API to set aria-level for the heading of the accordion panel

[jcgueriaud1]

What is the problem?

Currently the <vaadin-accordion-heading> has a role heading but you can't set the aria-level. So it's considered as level 2 which is not necessary the required level in a page.

Is it possible to add an API for it?

At least VoiceOver + Chrome and Safari are detecting Heading 2 in this page https://vaadin.com/docs/latest/example?embed=accordion-basic-wc.js&import=component/accordion/accordion-basic.ts

Browsers

• [X] Chrome
• [ ] Firefox
• [X] Safari
• [ ] Safari on iOS
• [ ] Edge

Screen Readers

• [ ] None
• [ ] NVDA
• [ ] JAWS
• [X] VoiceOver on MacOS
• [ ] VoiceOver on iOS