[4.0.0] Require users to explicitly define StringType, etc.

[falbrechtskirchinger]

I'm in the process of debugging #3602. It seems that it would be beneficial if users were required to explicitly tell the library what types are StringTypes, etc.

Just because a type is constructible from a string doesn't make it a string type. This can lead to the compiler selecting (or trying to select and failing) the wrong from_json function.

Leaving this here to be fleshed out at a later date.

[nlohmann]

Looking into it.

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This reply was drafted by Claude Code on behalf of @nlohmann, as part of a review of open discussions.