Return `dyn Iterator` instead of `Vec` for `SupportedContent`

Allows for cleaner representation of adding a single element: Some(T) instead of Vec<T> of length 1, but Vec<T> still works. This will probably end up being replaced by a more complex representation of how a particular element impacts the Rust AST, to represent things like inline style declarations (#show blocks).

Created by  finchie  on November 12, 2023
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