newrule(element_tabbable_unobscured): WCAG 2.2: 2.4.11 Focus not obscured (Minimum)

[shunguoy]

• New Rule(s): element_tabbable_unobscured

This PR is related to the following issue(s):

• Check WCAG 2.2 Focus not obscured requirement #1676
• <Help content>Check WCAG 2.2 Focus not obscured requirement #1679
• Add mapping to new WCAG 2.2 rule set for all existing 2.0/2.1 rules #1695
• [Bug] Duplication in Rule set headings #1725

Additional information can be found here:

Testing reference:

• first from the checker settings, select "WCAG 2.2 (A, AA)" guideline
  -- Then run the following test case, you should see an NR "Confirm that when the element receives focus, it is not covered or, if covered by user action, can be uncovered without moving focus" test/v2/checker/accessibility/rules/element_tabbable_unobscured_ruleunit/element_overlaid_hidden_default.html

I have conducted the following for this PR:

• I validated this code in Chrome and FF
• I validated this fix in my local env
• I provided details for testing
• This PR has been reviewed and is ready for test
• I understand that the title of this PR will be used for the next release notes.

[philljenkins]

I also reviewed the WCAG 2.2 rule set changes, and corrected a link to the Understanding page

[philljenkins]

And I fixed the 4.1..1 heading to include a description in #1725

[philljenkins]

The two changes made after Tom's review passed the build test.

[ErickRenteria]

Validated

[Seirdy]

Flattered that you linked me in the passing test case :3

I use your checker all the time so this feels nice.