ProgressBar: Add default tabIndex to make focusable

[tyxla]

What?

This PR updates the ProgressBar component to make it focusable.

cc @alexstine @andrewhayward for accessibility feedback.

Why?

While working on #53032 I noticed that the underlying progress element is not focusable and that makes it inaccessible. That meant we have to make it focusable so users with screen readers will be able to focus it and get it announced properly.

How?

This PR adds a default tabIndex of 0 to the underlying progress HTML element to make it focusable. We still allow the component user to override the value if they wish.

I'm also adding a unit test to verify the focusing works as expected. You can try removing the tabIndex and you should see the new test failing.

Testing Instructions

• This PR is part of Edit Site: Use progress bar for loading screen #53032, so you can test it there.
• Alternatively, spin up Storybook with npm run storybook:dev, load http://localhost:50240/?path=/story/components-experimental-progressbar--default
• Verify that the progress bar is focusable and announces properly that it's a progress bar in a loading state with a screen reader.
• Verify all tests pass - the change is covered by a new test I'm introducing.

Testing Instructions for Keyboard

Whether in the site editor or in Storybook, the element should be the sole focusable element in the canvas, so tabbing once should be enough.

Screenshots or screencast

None.

[mirka]

I believe what we're looking for (at least in the loading screen use case in #53032) is to associate the progress with the loading region.

I don't think <progress> elements are intended to ever be focusable, since they're not interactive.

[alexstine]

Sorry for causing a waste of time here. Turns out, the component has correct markup after the MDN example and no further work should be done to it.

https://developer.mozilla.org/en-US/docs/Web/HTML/Element/progress

I'll head back over to the other PR to continue discussion.

[tyxla]

Thanks for the info, @alexstine and @mirka 🙌 I'm happy to follow up on improving the component if anything occurs in the meantime.