Fix detecting invalid contents of block scalar headers

[fktn-k]

The current library can't detect invalid contents specified in block scalar headers, like the followings:

foo: |10 # the indentation must be between 1 and 9.

foo: >++ # too many chomping indicators

foo: |invalid # neither chomping nor indentation indicators must be a comment.

So, this PR has fixed the implementation for parsing block scalar headers and also added some relevant test cases.

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• [x] Changes are described in the pull request or in a referenced issue.
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