Serialize enum

[nugentcaillin]

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• added NLOHMANN_SERIALIZE_ENUM_STRICT to throw error on unrecognized enum values, solving 3992.
• Added new unit tests to verify behavior.
• updated documentation where NLOHMANN_SERIALIZE_ENUM to mention alongside warning that a throwing version exists - i didn't add a separate page for it as it is very close functionally to NLOHMANN_SERIALIZE_ENUM, but can add.

I wasn't able to call JSON_THROW inside detail/macro_scope.hpp, so this PR may have issues when exceptions are disabled - but am unsure if this is an issue if this macro has been added with the purpose of throwing - I'd appreciate some guidance on how to proceed here.