Add a way to indicate the semantics of ruby annotations

[w3cbot]

This is a tracker issue. Only discuss things here if they are a11y group internal meta-discussions about the issue. Contribute to the actual discussion at the following link:

§ w3c/html-ruby#24

[matatk]

Other issues/PRs of interest:

• Improving Accessibility of Ruby Annotations htmlwg#27
• A role for indicating whether a given ruby represents phonetics  aria#1620 (APA tracking issue at: ___)