Fix(Tearsheet): avoid focus traps with stacked tearsheets

[dcwarwick]

Closes #1332

This PR:

• adds tearsheets to the modal exit selector list so that stacking tearsheets do not "trap" the focus in the first tearsheet.
• catches focus entry into a non-topmost tearsheet and transfers focus to the topmost tearsheet.
• checks as each tearsheet becomes topmost that focus is within the tearsheet, and if not claims focus.

This should avoid the focus trap issue and also clean up the handling of focus when multiple tearsheets are being opened and closed.

What did you change?

Tearsheet.js, also added to the stacked tearsheet stories so the issue can be verified. Also updated the tests to expect warnings from Carbon -- these should eventually be removed (see carbon-design-system/carbon#10000).

How did you test and verify your work?

Ran storybook.

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[dcwarwick]

@matthewgallo @lee-chase I looked at the approach to this that replaced the ComposedModal with plain <div>s except for the topmost tearsheet -- ingenious, but I couldn't fix the animation problem. Basically, when React switches the rendering from the ComposedModal render method to the fragment is rerenders each side of an animation frame, so the transitions will not trigger correctly. I did explore first replacing the modal with <div>s in the OLD state, then after a delay switching the classes to try to get the transitions to occur, but it was becoming very fiddly.

Fortunately I found this alternative, which uses a rather obscure prop selectorsFloatingMenus. So obscure, in fact, that I think no one else has ever used it, because it actually has several bugs! I've opened carbon-design-system/carbon#10000 on Carbon to resolve those bugs. Fortunately, they only cause console warnings in non-production builds, so this is still a viable fix in the mean time. I've updated the tests to expect those warnings, and if/when Carbon is fixed we can remove those updates :-)

[matthewgallo]

Glad you found a work around for this! 🥇