[base] Removes invisible tabbable elements from `TrapFocus` component

[EthanStandel]

• [x] I have followed (at least) the PR section of the contributing guide.

Problem description

Described in issue #33380

Solution

Setting tabIndex={open ? 0 : -1} on sentinelStart and sentinelEnd elements so that they're only tabbable when the TrapFocus open prop is true. Thus removing the awkward experience where there are invisible tabbable elements.

[mui-bot]

Details of bundle changes

Generated by :no_entry_sign: dangerJS against bf6dd7d8a75c36394bef7c3ccde77a95a9d74644

[mnajdova]

cc @michaldudak would be great to have a second opinion on the changes. @samuelsycamore for reviewing the copywriting.

[michaldudak]

I haven't thought of such a use case before, but these changes look OK to me. As a follow-up, I think we'd have to rename the open prop. enabled would make more sense, especially for such cases.

[EthanStandel]

When I tried to resync the master branch of my fork the PR autoclosed! Reopening!

[EthanStandel]

@michaldudak I really like the idea of the prop being enabled, but I also know that change could be wide reaching. You're not recommending that change for this PR, are you?

[michaldudak]

No, not at all, this should be a separate PR that we can handle on our own.

[EthanStandel]

Same deal as before. Next time I won't open a PR off a fork from master. I'd change to another branch now but it seems like I'd have to open a new PR.

[EthanStandel]

Requesting a re-review from @samuelsycamore & @mnajdova ! 😄

Is that how I should do that? I legitimately don't know.