fixed parse error on explicit mappings as a block sequence entry

[fktn-k]

This PR has fixed parse errors on explicit mappings as a block sequence entry like the following valid YAML snippet:

- ? foo: 123
  : bar: true
- ? - baz
  : - null

The root cause was that the fkYAML parser just didn't assume explicit mappings as a block sequence entry.
So, implementation has been added for such nodes with some update of the test suite which validates the functionality.

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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%.
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[github-actions]

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[coveralls]

Pull Request Test Coverage Report for Build 9328722128

Details

• 13 of 13 (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 9326939126:	0.0%
Covered Lines:	3901
Relevant Lines:	3901

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