Fix comment prefix check

[fktn-k]

Although the YAML specification doesn't allow a comment to be followed right after non-break characters, the current lexer implementation doesn't detect such an invalid comment usage.

"foo"# invalid comment

This PR therefore fixes the implementation so the lexer emits an error when the '#' comment prefix doesn't meet all the following checks:

• it is the first character in the input buffer, or
• it follows after a white space character ( or \t) or a line break ('\n' or \r\n)

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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%.
• 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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