Fix parse error on flow container keys

[fktn-k]

This PR has fixed parse errors on flow container keys like the following valid YAML snippet:

{foo: bar}: 123
[true,false]: 3.14

The root cause is that the fkYAML deserializer wrongly assumed that flow containers weren't used as a mapping key.
To validate the fix, the test suite has also been updated by adding several relevant test cases for the deserializer.

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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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[github-actions[bot]]

:octocat: Upload Coverage Event Notification

Coverage data has been uploaded for the commit 530a897d44896bc0a01177bdd5ecd942b2a45dc1. You can download the artifact which contains the same file uploaded to the Coveralls and its HTML version.

Name	fkYAML_coverage.pr346.zip
ID	1538087095
URL	https://github.com/fktn-k/fkYAML/actions/runs/9240012472/artifacts/1538087095

[coveralls]

Pull Request Test Coverage Report for Build 9240012472

Details

• 148 of 148 (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 9237506365:	0.0%
Covered Lines:	3874
Relevant Lines:	3874

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