#240 Support non-string-scalar node keys in basic_node ctor with std::initializer_list

[fktn-k]

In this PR, non-string-scalar nodes (like mappings or integer scalar nodes) have been supported in the fkyaml::basic_node's constructor with std::initializer_list.
The unit test has also been modified according to the changes above.

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Pull Request Checklist

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• [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%.
• [x] The documentation is updated if you added/changed a feature.

Please don't

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[coveralls]

coverage: 100.0%. remained the same when pulling 619e6066e3aa5fc93eb238caa701ed1a4a1847e5 on feature/240_construct_mappings_with_initializer_list into 5694a455f3e6684fbd7a1d8b59060bdd70de90f1 on develop.