support flow line folding for single/double quoted scalars

[fktn-k]

This PR has added the feature of deserializing the flow line folding for single/double quoted scalars with the following restrictions:

• The flow line folding is being allowed for implicit keys despite that it's explicitly forbidden in the YAML specification. This is because of the lack of implementation to determine if scalars are implicit keys or not but such a logic is going to be implemented and the folding will become not allowed in the future.
• The flow line folding for plain scalars is not yet supported since the deserialization feature is not yet ready for that feature. It will, however, surely be supported in the future. Until the feature gets ready, please make such a scalar be either single or double quoted.

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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%.
• [x] 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[bot]]

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[coveralls]

Pull Request Test Coverage Report for Build 9147086497

Details

• 63 of 63 (100.0%) changed or added relevant lines in 1 file 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 9144792364:	0.0%
Covered Lines:	3752
Relevant Lines:	3752

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