Support Shiny Spinners

[astrowonk]

Shiny Spinners…

Not sure if this a feature request or a bug, but GT output in Shiny does not support spinners. See this example here.

(I'm not sure why, but it's rendering when run rather than wait for the buttons. Also, note that due to #567, I had to pin the great_tables version at 0.14 for the shiny live example but I see the same lack-of-spinner in 0.15 running locally with Shiny python on my Mac.)