[focus] FOCUSABLE_SELECTOR should exclude disabled buttons

[chloerice]

Right now, findFirstFocusableNode and findLastFocusableNode will focus disabled nodes. This is problematic because it breaks tabbability. Take for example the Polaris ActionList component when the first list item is disabled:

By changing const FOCUSABLE_SELECTOR = '...,button...'
to const FOCUSABLE_SELECTOR = '...,button:not(:disabled)...'

Tabbability remains intact and behavior is as expected:

I'm wondering if it makes sense to return a disabled node for any of the disableable selectors (a, input, select, textarea)? If not, when I make the PR I'll specify :not(:disabled) on those as well.

[svinkle]

a doesn't have a disabled attribute or state.

Adding an additional option for input, select, and textarea should do the trick.