Add format viewhelper to convert arrays to css custom properties

[schliesser]

Introduce a new ViewHelper (e.g. <f:format.cssVariables>) to convert array-structured data to CSS custom properties.

The ViewHelper recursively processes a given Fluid array, converting each key-value pair into a CSS variable.
The array keys are transformed into CSS variable names (e.g., :html:camelCaseKey becomes :html:--camelcasekey),
and the corresponding values are used as the variable values.
Only string values are rendered.

See: https://review.typo3.org/c/Packages/TYPO3.CMS/+/89729