Add "Name from Content" accname step for ::marker pseudo-element

[rahimabdi]

Closes https://github.com/w3c/accname/issues/203

Describe Change Here!

The accname algorithm should account for ::marker CSS-generated content. This PR for provides spec guidance on ensuring ::marker content (that is visible and where the current node supports it) is included in the accname computation.

Test, Documentation and Implementation tracking

Once this PR has been reviewed and has consensus from the working group, tests should be written and issues should be opened on browsers. Add N/A and check when not applicable.

• [ ] "author MUST" tests:
• [ ] "user agent MUST" tests:
• [ ] Browser implementations (link to issue or commit):
  • WebKit:
  • Gecko:
  • Blink:
• [ ] Does this need AT implementations?
• [ ] Related APG Issue/PR:
• [ ] MDN Issue/PR: