Indentation in multi-line double-quoted strings is not handled correctly

This issue was first alluded to in [0].
See this comment [1] for detailed description of required behavior.

As I understand, parser needs to detect the column of the opening double-quote character.

[0] https://bugs.opendaylight.org/show_bug.cgi?id=6180
[1] https://git.opendaylight.org/gerrit/#/c/41828/7/yang/yang-parser-impl/src/test/java/org/opendaylight/yangtools/yang/stmt/Bug6180Test.java@45