Separate YAML escaping/unescaping functionalities

[fktn-k]

This PR has created a new module yaml_escaper which executes YAML escaping/unescaping whose functionalities used to be implemented in multiple classes (especially in the lexer and the serializer).
Since this is just a refactoring, the library outputs will not change as I've checked them with the unit testing results.
Furthermore, the related test cases have been moved to the test source for yaml_escaper.

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• 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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[coveralls]

Pull Request Test Coverage Report for Build 8939448137

Details

• 272 of 272 (100.0%) changed or added relevant lines in 3 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 100.0%

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Totals
Change from base Build 8925561821:	0.0%
Covered Lines:	3599
Relevant Lines:	3599

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