[Feature request] Progress bar indeterminate

[guastallaigor]

What will it allow you to do that you can't do today?

It will make the progress bar indeterminate (constantly animated) if you pass a new prop indeterminate.

How will it make current work-arounds straightforward?

Just pass the new prop indeterminate, and remove the required attribute from the percent prop, because there will be no need in this case.

What potential bugs and edge cases does it help to avoid?

None, just a new feature for some loading cases.

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Note: If you accepted this, I've already have the code to make a PR, just let me know. Thanks.