added config file for AppVeyor

[fktn-k]

This PR added a config file for AppVeyor.
The configuration includes build/unit-testing on Visual Studio 2015 and Visual Studio 2017 which haven't been supported since GitHub Actions doesn't provide runner images for those environments.
Since any build/unit-testing have not been triggered, the documentation is intentionally left untouched.

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Pull Request Checklist

Read the CONTRIBUTING.md file for detailed information.

• [x] Changes are described in the pull request or in a referenced issue.
• [x] The test suite compiles and runs without any error.
• [x] 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.

[coveralls]

coverage: 100.0%. remained the same when pulling 9b851cc0549af17f8245bb1c69e96c185d9ceea9 on feature/add_supported_msvc_versions into ff545755cd5759452a952d76df615b2009952cc1 on develop.