revealjs - set code-font-size as em value for variable default

[cderv]

This fixes #11373 by adapting previous changes in #11028 and then #11237. The changes initially introduced for Brands were making code font size default being pixels, while previous is was set to em and so code font size was adapting based on context like headers.

This PR is making code font size being based on em like Quarto 1.5 while keeping the customizable values. Defaults are chosen like this

• $code-font-size in Revealjs is em units by default which seems more coherent with its bootstrap counter part, also as em. Value is 1em for revealjs.
• $code-block-font-size is set to 0.55em - same as previous Revealjs default, smaller than bootstrap to save some slide space.
• $code-inline-font-size is set to 0.875em - same as bootstrap

This also simplifies previous logic with smaller context (like callout or smaller slides), and so previous adaptation from previous followed up PR are removed (and test adapted).

I believe this is closer to default RevealJS and previous Quarto 1.5 style, while keeping the $code-font-size

And it is probably better for brand for size being passed to code-font-size in same units for revealjs and bootstrap.

Quarto example has been updated with this PR

• https://examples.quarto.pub/revealjs-default-callouts-styles/#/title-slide