Card stack: attribute name should be tag -> tags?

[damithc]

The name tags seem more consistent with the purpose?

However, MarkBind already supports an attribute called tags https://markbind.org/userGuide/usingPlugins.html#plugin-tags So, we might need a separate term altogether e.g., categories or labels.

Although this will be a breaking change, perhaps we can sneak it into v6.1 as this feature is brand new and unlike to have been used yet.