add 'summary' element to list of focusable selectors

[VinceMalone]

What:

add summary element to list of focusable selectors

Why:

in all major browsers, the summary element is focusable via keyboard (when it's a direct descendent of a details element)

making this change will allow user.tab() (via getTabDestination()) to focus summary elements; which matches the behavior found in all major browsers.

• in chrome, firefox, and safari: document.activeElement results in the <summary> element
• in devtools, the summary element is consistently labeled as focusable
  • in chrome, the element is given a more abstract role called "DisclosureTriangle", but is always marked as focusable ("Details" is not)
    
    
  • in firefox, the same is true
    
  • unfortunately, safari doesn't provide a way to inspect its accessibility tree, but it matches the behavior of other vendors via document.activeElement

How:

Checklist:

• Documentation
• Tests
• Ready to be merged
  

what about #947 and what about details being marked as "interactive content" per the spec, but summary technically is not?

i understand there's some nuance to <details> without a <summary> and that details is the one that marked as "interactive content", but what reflects reality is that...

• all major browsers treat the summary element as interactive (see above); and never the details element itself
• MDN reports an implicit role of "button" for <summary>, where as <details> has an implicit role of "group" (this more or less matches what's seen in devtools)
• even if there's more nuance in addition to a focusable summary element (such as details without a descendent summary), this doesn't change the most common case where details > summary is focusable

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