Add normative text on calculating aria-level, aria-posinset, and aria-setsize

[smhigley]

Closes #1194

As discussed in the issue, this PR adds normative wording on requiring user agents to calculate aria-level, aria-posinset, and aria-setsize. It also adds a section outlining how to compute each attribute, based on the current Core AAM wording that will be removed after this is merged.

PR tracking

Check these when the relevant issue or PR has been made, OR after you have confirmed the related change is not necessary (add N/A). Leave unchecked if you are unsure. Read the Process Document or Test Overview for more information.

• [ ] Related Core AAM Issue/PR:
• Related AccName Issue/PR: N/A
• Any other dependent changes? N/A

Test, Documentation and Implementation tracking

Once this PR and all related PRs have been been approved by the working group, tests should be written and issues should be opened on browsers. Add N/A and check when not applicable.

• Related APG Issue/PR: N/A
• MDN Issue/PR: N/A
• "author MUST" tests: N/A
• [x] "user agent MUST" tests: issue created at https://github.com/web-platform-tests/interop-accessibility/issues/99
• [ ] Browser implementations (link to issue or when done, link to commit):
  • WebKit:
  • Gecko:
  • Blink:
• Does this need AT implementations? N/A

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