Add std::error_code and error category constructors to system_error

[Seele-Official]

Summary

This PR extends cpptrace::system_error with constructors accepting:

• std::error_code
• An error value together with a const std::error_category&
• Both forms with an optional custom message

This brings cpptrace::system_error closer to the interface provided by std::system_error.

Details

The existing (int, std::string&&) constructor has been preserved for source compatibility. Its implementation now delegates to the new std::error_code constructor using std::generic_category(), preserving the existing behavior while avoiding duplicate initialization and message-formatting logic.

I intentionally did not reproduce the complete set of message overloads provided by std::system_error, which accepts both const char* and const std::string&. The existing cpptrace exception hierarchy consistently accepts messages as std::string&&, so this PR follows that convention and keeps the change focused on error-code and error-category support.

In the longer term, I think it would be useful for cpptrace's exception types, not only system_error, to support const char* and const std::string& as well (refer to https://en.cppreference.com/cpp/header/stdexcept). Although string literals can currently be passed through an implicit conversion, doing so requires constructing a temporary std::string. Dedicated overloads would improve compatibility with the standard exception types, make the API more convenient to use, and may avoid unnecessary temporary construction or moves.

Closes #326