Allow backslashes in plain/single-quoted scalars

[fktn-k]

The current fkYAML parser throws a parse_error exception when a plain/single-quoted scalar contains backslashes () which are allowed in the YAML spec.
For example, the following valid YAML snippet cannot be parsed correctly because of that wrong implementation:

plain: foo\bar.yaml
single quoted: 'foo\bar.yaml'

This PR has fixed the above issue and updated some related test cases which have wrong expectations.

---

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]]

:octocat: Upload Coverage Event Notification

Coverage data has been uploaded for the commit 4030626ef09848c488c1277e84630e3f282e3f7e. You can download the artifact which contains the same file uploaded to the Coveralls and its HTML version.

Name	fkYAML_coverage.pr329.zip
ID	1455580300
URL	https://github.com/fktn-k/fkYAML/actions/runs/8873963110/artifacts/1455580300