Add 2.1.3 to iframe focusable rule

[WilcoFiers]

Closes #2026

• This added 2.1.3 Keyboard (No Exceptiosn) (AAA) to this rule.
• This seemed slightly better to me.

IMPORTANT: This is going to cause implementors to be inconsistent. That's why I'm using a 2 week review. It is incorrect for this rule not to fail 2.1.3 for the same reason that Scrollable content can be reached with sequential focus navigation fails 2.1.3. The only difference between 2.1.1 and 2.1.3 is an exception for path based input. That's not the case for this iframes or scrollable regions.

Need for Call for Review: 2 weeks

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[Jym77]

Isn't that a case where Scenario 1 (the rule is not as strict as a requirement) of Secondary requirements applies?

That is, anything that fails 2.1.1 also automatically fails 2.1.3, similarly to the 1.4.3/1.4.6 situation.

Additionally, 2.1.3 is a AAA criterion, so having this as a primary requirements means that tools that only check for level AA will never be able to have a consistent implementation of the rule (which was pretty much what led us to introduce secondary requirements for the 1.4.3/1.4.6 or 2.4.6/2.4.9 cases: initial PR, linking to more issues in the wcag-act-rules repo)