🪈 propagate class value through to react on container type directives

[stevejpurves]

Motivation for this PR is to enable more control over final rendering of some container directives by allowing authors to use the class option to add classes to the final container component that is rendered on page.

When teamed with user defined stylesheets this will be very powerful in the core themes and is immediately useful when using current/known tailwind classes, and in custom themes.

tested with:

# testing class flows

this is the main content this is more main content this is more main content this is more main content this is more main content this is more main content this is more main content

:::{aside}
:class: test-aside
this is an aside this is an aside this is an aside this is an aside this is an aside this is an aside this is an aside
:::

this is more main content this is more main content this is more main content this is more main content this is more main content this is more main content this is more main content this is more main content this is more main content

:::{code} javascript
:class: test-code
const a = 1234
:::

this is more main content this is more main content this is more main content this is more main content this is more main content this is more main content this is more main content this is more main content this is more main content

:::{iframe} https://mystmd.org/sandbox
:class: test-iframe

:::

this is more main content this is more main content this is more main content this is more main content this is more main content this is more main content

:::{image} banner.png
:class: test-image
:::

:::{figure} banner.png
:class: test-image-figure
:::

this is more main content this is more main content this is more main content this is more main content

:::{figure} ./videos/links.mp4
:class: test-video
An embedded video with a caption!
:::