implement `processing` for the n-progress `circle`

[wc-matteo]

This function solves the problem (这个功能解决的问题)

being able to show processing with the type circle

Expected API (期望的 API)

no api changes

see: https://codesandbox.io/s/dreamy-williamson-rkcdu

[equt]

Could you please provide a more clear description of what this could do, especially where it differs from the previous issue #508

[wc-matteo]

processing, as you've implemented it, makes it clear that the progress indicator with a definite state (i.e. a state with some percentage of completion) is still active and doing something even though the percentage is not incrementing. The issue #508 asks for an indefinite state (i.e. a state where there is not beginning or end) mode.