Scalar\String_ and Scalar\Encapsed now have an additional "kind"
attribute, which may be one of:
* String_::KIND_SINGLE_QUOTED
* String_::KIND_DOUBLE_QUOTED
* String_::KIND_NOWDOC
* String_::KIND_HEREDOC
Additionally, if the string kind is one of the latter two, an
attribute "docLabel" is provided, which contains the doc string
label (STR in <<<STR) that was originally used.
The pretty printer will try to take the original kind of the string,
as well as the used doc string label into account.
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