Add hour12 date/time override option

[eemeli]

This is the part of the changes originally proposed in #911 that was deemed as immediately mergeable on today's call.

Perhaps a little pre-emptively, it adopts the nomenclature to be introduced in #946.

This PR is expected to be followed up with separate PRs for the RECOMMENDED override options.

[mihnita]

Looking at the whole file, it looks like the way "Date/time override options" is added to :datetime and :time is inconsistent.

:datetime has it in text, but not in the list of options:

The :datetime function can use either the appropriate style options or can use a collection of field options (but not both) to control the formatted output. Date/time override options can be combined with either style options or field options.

Which is kind of hard to discover.

:time has it in the bulleted list of options

:date has it in the bulleted list of options

But it should not have it at all.
Because right now the only Date/time override option is "hour12"
Which does not apply to dates (they don't have an hour field).

[aphillips]

This meets fast-track requirements and was approved in the 2024-11-18 call previously.

[mihnita]

was approved in the 2024-11-18 call previously

Allowing hour12 on :date is clearly wrong.
We do that just because was approved for fast-track?

The fix for that would be simple: remove it from :date

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I can live with the option "hidden in text" for :datetime
It is a bit hard to discover, but not wrong.

[mihnita]

Remove this line.
The overrides right now only support hour12, which is not valid on dates.