add option to isFocusable to take into account negative tabindex

[straker]

The isFocusable function currently does not take into account negative tabindex (and for the default use case it shouldn't take into account negative tabindex). This causes us to pair the function call with a check on tabindex to ensure that the element is currently keyboard focusable and not programmatically focusable.

const tabindex = vNode.attr('tabindex');
if ((isNaN(tabindex) || tabindex >  -1) && isFocusable(element)) {
  // ...
}

We should add an option to the function that enables looking for negative tabindex and changes the return to false for the element. Something like programmatically: false or noNegativeTabindex: false.

if (isFocusable(element, { programmatically: false }) {
  // ...
}

[padmavemulapati]

Validated using latest develop branch code base,
If negative tab index is there within the frame it is failing but if it is in focussible area, then it is ignoring
Script:

<script src="/axe.js"></script>