Add MAGIC_ENUM_USE_STD_MODULE option to use standard library module.

[stripe2933]

This PR adds two functionalities:

• When user declares MAGIC_ENUM_USE_STD_MODULE macro, standard library header inclusions are skipped, and it uses import std; instead. It prevents multiple header compilation and improves the build time. This only affects to the codebase when user use magic_enum.cppm module and declare the macro. Similar structure to argparse.cppm.
• Export std::formatter template specialization, which was missing in PR #343.

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Fix: Commit a3934f7

• Move magic_enum.cppm module file to module directory, since module is not header file and doesn't match to the meaning of include.