Allow dates and filesizes to be converted to headers by group-by (and headers, I suppose)

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Related problem

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Describe the solution you'd like

The main reason I want this is so that ls | group-by size (and such) can "just work" without needing to explicitly convert the entire column to string. I think this is appropriate because filesizes and dates are both "numeric-ish" types that aren't complex or ambiguous to serialise (unlike data structures, closures, etc).

To turn these to headers, these values should be converted to their to nuon serialisation, which are unambiguous representations unhindered by the user's config or any relative rendering.

I don't really care about headers that much, but, since it now has type-checking (#6918) I think these commands should both have consistency in what types are "header-able". (Desirable Idea: try and abstract this out and make "can_be_header" a method of Value. Maybe don't call it that exactly, though…)

Describe alternatives you've considered

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Additional context and details

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