Add `write()` export

[K20shores]

See #268. Add include/mechanism_configuration/exporter.hpp.

Acceptance criteria

• void write(const Mechanism&, const std::filesystem::path&) compiles
• .yaml produces YAML; .json produces JSON; unknown suffix is an error
• Round-trip: parse(path) then write(out) then parse(out) produces equal Mechanism

Ideas

• Unknown suffix must error; silent wrong-format output is worse than a clear error
• might have to do some tricks to emit json, but seems possible?
  • stackoverflow answer
  • or could always add nlohmann::json as a dependency for writing out