Flex content in simple callout in revealJS does not spread full with

[cderv]

One improvment to add

It seems we never styled correctly the simple callout content.

FWIW in HTML it looks like

It applies to version with icon too

And version with title

Originally posted by @cderv in #11251 (comment)

This is probably related to callout-body-container rule with flex-grow: 1 that never applied in revealjs

Full examples of newest style with the problem at
https://examples.quarto.pub/revealjs-default-callouts-styles