In this PR, parsing multiline plain (unquoted) scalars has been implemented, which was described as a restriction in the PR #344

The flow line folding for plain scalars is not yet supported since the deserialization feature is not yet ready for that feature. It will, however, surely be supported in the future. Until the feature gets ready, please make such a scalar be either single or double quoted.

Before this PR, multiline plain scalars seemed to be parsed with no error but the parse result was just incorrect since its data structure was different from the expected one. (I know this was terrible in debugging...)
The library can now successfully parses such scalars and the result has its correct data structure.
We can simply pass this YAML

key: this line
  is a multiline
  plain scalar.

to the fkyaml::node::deserialize() function, and the function then interprets the input as if the following YAML is passed.

key: this line is a multiline plain scalar.

Note that, although multiline scalars (either plain or single/double quoted) cannot be used as an implicit mapping key as stated in the YAML specification (7.3. Flow Scalar Styles), the library currently allows for such an invalid syntax like this:

this implicit
  multiline
  scalar: should not be allowed

This will be fixed in another PR for sure.

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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.
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