Restore `rb` and `rtc`

[frivoal]

#253 removed the rb and rtc sub elements of ruby, because they are been marked obsolete in the HTML specification. https://www.w3.org/TR/html-ruby-extensions/ is restoring them, and so they should be restored in accessibility mappings as well.

(See the introduction and appendix B for rationale.)

In terms of roles, I believe the following ones would be correct:

• for rb

  • MSAA+ IAccessible2: Role: ROLE_SYSTEM_TEXT; IA2_ROLE_TEXT_FRAME
  • AX: AXRubyBase

• for rtc: it's container for rt elements, and occasionally anonymous text nodes with the same semantics as rt, so probably the same roles as rt would seem to make sense.

This blocks w3c/html-ruby#45

[spectranaut]

Just for reference: w3c/html-aria#572