Use std::unreachable() or similar compiler specific extensions for dead but necessary code

[fktn-k]

The current library deliberately implements some dead codes to suppress warnings against default switch-cases. However, such codes are considered as a possible path from the compiler's view as much frequently as the other "valid" cases.
So, in this PR, such codes have been replaced by calling a function which calls one of the following functions (upper one has more priority) so that a compiler can know the codes are actually unreachable and can be optimized away:

• std::unreachable() (since C++23)
• __assume(false) (MSVC)
• __builtin_unreachable() (other compilers like GCC or Clang)

Along with the above change, FK_YAML_HAS_CXX_23 macro has been added to check if the active C++ standard is 23 or greater. See here for its usage. The library currently has no implementation specific to C++23 features but detection of std::unreachable().

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Pull Request Checklist

Read the CONTRIBUTING.md file for detailed information.

• Changes are described in the pull request or in a referenced issue.
• The test suite compiles and runs without any error.
• The code coverage on your branch is 100%.
• The documentation is updated if you added/changed a feature.

Please don't

• The C++11 support varies between different compilers and versions. Please note the list of supported compilers. Some compilers like GCC 4.7 (and earlier), Clang 3.3 (and earlier), or Microsoft Visual Studio 13.0 and earlier are known not to work due to missing or incomplete C++11 support. Please refrain from proposing changes that work around these compiler's limitations with #ifdefs or other means.
• Please refrain from proposing changes that would break YAML specifications. If you propose a conformant extension of YAML to be supported by the library, please motivate this extension.
• Please do not open pull requests that address multiple issues.

[github-actions]

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