Allow trailing comma in flow mapping/sequence

[fktn-k]

The current parser didn't allow trailing commas in flow mappings/sequences like the following valid YAML snippet:

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

The YAML spec allows both putting/omitting trailing commas in flow mappings/sequences.
So, this PR has modified the implementation to allow putting trailing commas by modifying state management for flow contexts.

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Read the CONTRIBUTING.md file for detailed information.

• 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.
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[github-actions]

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ID	1605953302
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[coveralls]

Pull Request Test Coverage Report for Build 9534915247

Details

• 30 of 30 (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 9532939917:	0.0%
Covered Lines:	3932
Relevant Lines:	3932

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💛 - Coveralls

[coveralls]

Pull Request Test Coverage Report for Build 9535200345

Details

• 44 of 44 (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%

---

Totals
Change from base Build 9532939917:	0.0%
Covered Lines:	3944
Relevant Lines:	3944

---

💛 - Coveralls

[coveralls]

Pull Request Test Coverage Report for Build 9535291146

Details

• 44 of 44 (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%

---

Totals
Change from base Build 9532939917:	0.0%
Covered Lines:	3944
Relevant Lines:	3944

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💛 - Coveralls