fix(scrollable-region-focusable): exclude overflow:hidden as not scrollable

[straker]

An element with overflow: hidden is not scrollable except through JavaScript elm.scrollTop. This prevents common CSS techniques to hide elements from visual users from failing the rule.

e.g.

.sr-only {
  border: 0; 
  clip: rect(0 0 0 0); 
  height: 1px; 
  margin: -1px;
  overflow: hidden;
  padding: 0;
  position: absolute;
  width: 1px;
}

Linked issue: #1582

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• Code is reviewed for security by: @JKODU